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CHAPTER 3

APPARATUS FOR EMG MEASUREMENT

This chapter discusses the design and material used for the EMG measurement

device. The device is basically an EMG amplifier. For our future task, EMG data will be

used for classification of prehensile EMG patterns. After feature extraction, the adequate

control signals will be generated, and then sent to the robot hand. The device should be able

to capture the EMG signals from four surface electrodes. These signals will be recorded for

thumb, index finger, middle figure, and a combination of ring and little fingers. The device

consists of four main parts: electrodes and extensions, preamplifiers, filters, and amplifiers

with bias adjustment. To store and display the data on a computer, we will need an A/D

converter (analog to digital converter) and user programming interface. This requires the

amplification of signals to the TTL level (which ranges between -5 volts and +5 volts).

Otherwise the computer would not be able to recognize the signals.

Figure 3.1 shows the stages of an EMG measurement or recording. When a muscle is

contracted, the electrode picks up an EMG signal. Then the preamplifier boosts the signals

high enough to prevent electrical interference. The preamplifier also filters the noise. After

that the additional amplifier increases the amplitude of signals to the TTL level and adjusts

the bias or offset if necessary. Finally A/D converter converts signals into digital form to

allow the EMG Capture (EMGC) program to read and store EMG data in the file. The

EMGC program is described in more detail in Chapter 4.

The device will have four input channels, and one set of outputs which will be

connected to the analog capture card (A/D converter). The finished product looks like a

plastic box that contains four input phone jack connectors, one output RJ-45 connector, a

couple of adjustment screws and a power switch.

When performing a grasping experiment, the electrodes will be placed on the area of

the muscles as shown in Figure 3.2. The EMG amplifier device will be placed under or over

the forearm.

21

Figure 3.1: EMG measurement stages.

Figure 3.2: EMG amplifier device.

3.1 ELECTRODES AND EXTENSIONS An EMG amplifier is designed to be used with a skin surface electrode. This type of

electrode is defined as a bipolar electrode. The surface electrodes are not expensive. The

problem with skin surface electrodes is that they create sometimes an unstable contact. An

unstable contact causes potential motion artifacts. It could also add thermal noise to an EMG

signal. A high impedance electrode could help prevent thermal noise problem. To avoid

unstable skin contact the electrode must be placed firmly to the skin.

To secure the electrode contact the electrode that has an adhesive surface is selected

for this EMG amplifier. The electrode used in this project is a Square Cloth [15] electrode

which is a pad that has dimension of 0.875 inches. A Velcro strap is used to secure the

electrode to the skin for additional contact stability.

22

The extension is a stereo phone wire soldered to a snap-on and sewed on a piece of

plastic with a Velcro strap as shown in Figure 3.3. A Velcro strap is used to wrap around the

forearm to stabilize the contact of the electrode. A snap-on is used for holding the electrode.

A signal will be transmitted to the preamplifier by the stereo phone wire.

Figure 3.3: Extension.

Using a stereo phone wire is fitted to the design of our EMG amplifier. The ground

conductor of the stereo phone wire shields the input of the preamplifier circuit from noise. It

is connected to the body reference circuit as shown in Figure 3.6 (shield, discussed in Section

3.2.2). The stereo phone wire consists of three conductors. They are commonly used in the

left and right speakers of basic stereo. The two speakers share the same common ground

conductor. As in the design of our EMG amplifier, a preamplifier uses a differential amplifier

circuit which needs two positive and negative inputs. One of the body reference circuit inputs

is a shield wire as shown in Figure 3.6. This shield provides protection from noise. The left

and right conductors of the stereo phone wire are preamplifier inputs. A ground conductor is

for shielding. In addition, a stereo phone wire is flexible enough to do a grasping experiment

without changing the orientation of the electrode contact with the skin. The length of the wire

should be short. The longer wire provides more chance of picking up noises. However, the

wire should have enough slack to allow plug-in into the EMG amplifier.

The total number of electrode contacts to be used in this EMG amplifier is nine, with

eight electrode contacts used for the four input extensions (two differential contacts for each

electrode extension), and another one used for the body reference extension. For the input

extension electrode, the contacts are placed about one inch from each other as shown in

23

Figure 3.4. Since the pad has a dimension of 0.875 inches, they cannot be placed closer to

each other, because the pads must not touch each other.

Figure 3.4: The distance between electrodes.

3.2 PREAMPLIFIER As explained earlier, EMG signal is so small that computer could not read it. While

the amplitude of the signal is between 0 to 10 millivolts (peak-to-peak), or 0 to 1.5 millivolts

(rms), the usable frequency of an EMG signal is ranging between 50-150 Hz [7]. At this

state, we need a huge gain (about thousand times) to boost the EMG signal without changing

phase or frequency of the signal. This preamplifier uses a typical differential amplifier

circuit, which contains two inputs (positive input and negative input). The differential

amplifier circuit subtracts two inputs and amplifiers the difference.

To get the right level of the input signal, we need a body reference circuit which

works as a feedback from the inputs. Whenever the body temperature changes or signal

changes due to noise introduced by the body, this body reference will help maintain the

correct level of signal. In each input channel, there is one body reference feedback. However,

with four channels, we could use common body reference feedback by averaging body

references from all four channels. The following section will explain the specific details of

the preamplifier.

3.2.1 Power Supply The Power supply unit provides positive voltage, negative voltage, and ground for all

components in the circuit. The 9-volt battery is suitable to supply voltage for all the

components because it is easy to find and is cost-effective.

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In order to create positive and negative power supply voltage, two 9-volt batteries

were connected in series. The ground line was placed at the junction between the two

batteries. Therefore the +9 volts and –9 volts provided the source for the power supply.

The capacitors function to stabilize the power supply current. Thus, they prevent

dramatic change in current. This is necessary because when the components are too distant

from the power source, oscillation of the current could develop. Such an oscillated current is

unsuitable for an integrated circuit (IC) [3]. The two capacitors (C1 and C2) are in parallel

connection with the 9-volt batteries as shown in Figure 3.5. These capacitors compensate

oscillations.

Figure 3.5: Power supply unit circuit.

3.2.2 Preamplifier and Body Reference Circuit A suitable operational amplifier (op-amp) for this type of application is the

Instrumentation Amplifier. This type of op-amp usually provides excellent accuracy because

it provides high bandwidth even at high gain. The EMG amplifier used a BURR-BROWN,

INA2128 chip for the preamplifier and OPA2604 for body reference circuit, as suggested by

the application information data sheet of INA2128 [4]. The data sheet of INA2128 and

OPA2604 [4] [5] is shown in Appendix B.

INA2128 is a dual op-amp. Therefore, only two chips were used for four channels.

OPA2604 is also a dual op-amp. This saved space on the printed circuit board (PCB).

25

Figure 3.6: Preamplifier with body reference circuit.

Figure 3.6 shows the preamplifier circuit and the body reference circuit from only one

channel. The preamplifier is a differential amplifier circuit. The EMG amplifier has four

channels. The preamplifier needs a huge gain of thousand times. As shown in [4], the

equation for gain is:

501G

kGainR

= + (3.1)

where / 2 1 2GR R R= = , for gain = 1,000 can be determined directly from equation (3.1)

1 50 1 50 25.0252 2 ( 1) 2 (1000 1)GR k k

Gain= × = × =

− −

After calculation, / 2GR is about 25 ohms. However, the closest value of resistance available

in market is 22 ohms. Thus, the resistor value of 22 ohms was used for R1 and R2 as shown

in Figure 3.6.

Substituting / 2 22GR = ohms into equation (3.1) gives:

501 1137.3642 22kGain = + =

×

Therefore the actual gain for the preamplifier is about 1137. This is sufficient for our

purpose.

3.2.3 Averaging Body Reference Circuit Our EMG amplifier has four channels; therefore, there are four preamplifiers and four

body reference circuits. However, these four body reference circuits could be combined into

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one common body reference circuit. This reduces the number of electrode contacts from four

to one. To create the common body reference circuit, an inverting summing is used. For

summing amplifier, the value of each resistor should be calculated. With the right ratio of

resistor values, the inverting summing amplifier would provide an average output result.

Figure 3.7: Inverting summing amplifier circuit.

Figure 3.7 shows the inverting summing amplifier circuit for four inputs.

Resistor BR is a compensating resistor. It is placed to ensure that both inputs of the op-amp

(positive and negative inputs) have similar resistance to ground. This helps to minimize

problems caused by op-amp’s bias current. The value of resistor BR is equal to the parallel

combination of FR and all of the input source resistors. The output voltage (Vout ) can be

computed as follows [37]:

For independent 1R , 2R , 3R , 4R , and FR :

31 2 4

1 2 3 4out F

VV V VV RR R R R

⎛ ⎞= − + + +⎜ ⎟

⎝ ⎠

For 4321 RRRR === , and FR independent

( )1 2 3 41

Fout

RV V V V VR

= − + + + (3.2)

From equation (3.2), average output can be taken by giving the correct ratio to 1/FR R ; in our

case 1/FR R is approximately equal to1/ 4 . However, using an inverting summing amplifier

op-amp, the output still has an opposite sign to the input. To change the sign for the output,

another circuit is needed.

27

Figure 3.8: Sign changing circuit.

Figure 3.8 shows the sign changing circuit. The sign changing circuit is actually built

from an inverting amplifier circuit which has a gain equal to one. The output of an inverting

amplifier circuit can be determined by [12]:

2

1out

RV VinR

= − (3.3)

if 12 RR =

outV Vin= −

To create the average body reference, the BURR-BROWN, OPA2604, dual op-amp

was used to construct the inverting summing amplifier circuit and the sign changing circuit.

Figure 3.9: Averaging body reference circuit.

Figure 3.9 is the averaging body reference circuit, a combination of an inverting

summing amplifier circuit and sign changing circuit. Ideally, using a resistor ratio of 1/ 4 , the

inverting summing amplifier yields the output as an opposite sign from the input from the

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body references of four channels. Using the resistor value from Figure 3.9, we can compute

outV at point A using equation (3.2) by:

( )1 2 3 41

4.7outAkV Bodyref Bodyref Bodyref Bodyrefk

= − + + + (3.4)

The result of VoutA is the input for the sign changing circuit. Using equation (3.3), the

voltage at point B is:

( )

( )

( )

1 2 3 4

1 2 3 4

111 11 4.7

14

outB outAkV Vkk k Bodyref Bodyref Bodyref Bodyrefk k

Bodyref Bodyref Bodyref Bodyref

= −

⎛ ⎞⎛ ⎞= − − + + +⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

≈ + + +

As a result, at point B, we get the average body reference of four channels which has

the same sign as input. Therefore, this point can be used for a common body reference

electrode.

3.3 FILTER The filters were applied after the output of the preamplifier, so that they help to

prevent noise that has been amplified by the preamplifier. The filter also helped to sink any

DC current that could cause bias for the signal. In our EMG amplifier, a simple RC high pass

filter circuit was used. Since the usable energy of the EMG signal is dominant within the

range of 50-150 hertz, a cutoff frequency of about twelve hertz for a high pass filter was

selected because it will not reject the necessary information of the EMG signal. To get a

twelve hertz cut off frequency, a resistor value of 91 kiloohms, and capacitor value of 150

nanofarads were selected to connect an RC high pass filter circuit as shown in Figure 3.10.

Figure 3.10: RC high pass filter with cutoff frequency of about 12 Hz.

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The cutoff frequency of the RC filter can be computed from [12]:

12cutofffRCπ

⎛ ⎞= ⎜ ⎟⎝ ⎠

. (3.5)

The cutoff frequency of our RC filter can be computed by using equation (3.5). With a

substitute resistor and capacitor value as given in Figure 3.10, the cutoff frequency of the

filter becomes:

Hz

nFkfcutoff

1266.11

1509121

≈=

⎟⎠⎞

⎜⎝⎛

××=

π

3.4 GAIN AND BIAS ADJUSTMENT The amplifier and bias adjustment could be considered as a second stage for the EMG

amplifier. The amplifier and bias adjustment provide an ability to adjust or correct the output

signals in some circumstances. For example, the amplitude of amplified signal from the

preamplifier is not high enough for an A/D converter. Or, the amplified signal still has a bias

or offset. Therefore, these problems can be resolved by using of the gain and bias adjustment.

In our EMG amplifier device, each channel has an individual gain adjustment unit

and an individual bias adjustment unit. In other word, the amplification of the signal made

for channel one will not affect the other channels. Within the same channel, the gain and bias

adjustment are related since the output of an amplifier is the input to the bias adjustment

circuit. Therefore, when doing the calibration, one should take this fact into account. The

calibration procedure is explained more in section 3.7.

There is a limitation of the gain and the bias adjustment. The amplifier in our EMG

device can amplify about 21 times. The bias adjustment can adjust the signal up or down by

the level of positive nine volts, or negative nine volts. However, by the nature of op-amp, the

output from the op-amp can not be more or less than the power supply voltage. For example,

if in a given two volts of input are fed to an amplifier which has a gain equal to three times

(gain is equal to three), and then the offset is increased by positive two volts, the final output

from calculation will be equal to eight volts[ ](2 3) 2× + . If the power supply is 9+ volts, and

9− volts, op-amp can produce eight volts. This is acceptable. However, if the gain of an

30

amplifier is changed from three to four times, with the same input level, the final output from

the calculation will be equal to ten volts[ ](2 4) 2× + . Because of the characteristic of the op-

amp, nevertheless the op-amp will give the output of nine volts or less, due to the internal

voltage drop of the op-amp.

3.4.1 Amplifier (Gain) The amplifier in our EMG device uses basic non-inverting amplifier circuit. The input

of the amplifier is the output from the filter. To be able to adjust the gain, potentiometers

(adjustable resistors) were used to connect the circuit. The highest gain of the amplifier is

designed to be approximately 20 times or 21 times in theory.

Figure 3.11: Non-inverting amplifier circuit.

Figure 3.11 demonstrates non-inverting amplifier circuit. outV and gain can be

determined by [12]:

3

1

1out inRV VR

⎛ ⎞= +⎜ ⎟

⎝ ⎠,

3

1

1 RGainR

⎛ ⎞= +⎜ ⎟⎝ ⎠

. (3.6)

The compensation resistor Rs is placed to reduce the error in the output voltage. The

error is caused by the voltage drops resulting from the op-amp’s input bias current [12]. The

value of Rs is usually equal to the parallel combination of 3R and 1R .

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In our EMG amplifier, the BURR-BROWN, OPA2604 chip is used to construct a

non-inverting amplifier circuit. The OPA2604 chip is a dual op-amp; therefore, one op-amp

was used for gain circuit and another op-amp was used for bias adjustment circuit.

Figure 3.12: Amplifier circuit with gain adjustment.

Figure 3.12 shows the circuit of amplifier for one channel. 34R is a compensation

resistor, which is equal to the parallel combination of 40R and 35R . 40R is a potentiometer or

adjustable resistor. Therefore, when 40R is adjusted to 0, there is no gain (gain =1). On the

other hand, when 40R is adjusted to the maximum of 200 kiloohms, the gain is equal to 21.

The calculation can be done by using equation (3.6).

At 040 =R :

40

35

1

0110

1

RGainR

k

⎛ ⎞= +⎜ ⎟⎝ ⎠

⎛ ⎞= +⎜ ⎟⎝ ⎠

=

At kR 20040 = :

40

35

1

200110

21

RGainR

kk

⎛ ⎞= +⎜ ⎟⎝ ⎠

⎛ ⎞= +⎜ ⎟⎝ ⎠

=

32

Computing the value of 34R :

40 3534

40 35

200 10200 109.52410

R RRR Rk kk kk

k

=+

×=

+=≈ Ω

3.4.2 Bias Adjustment Bias adjustment circuit is used to increase or decrease the reference level of the

signal. Normally, the reference level of the signal is at ground line or zero volts. However, if

the reference level is not at ground line, or the desired reference level is needed, the bias

adjustment enables a user to adjust the reference level to the desired level.

Figure 3.13: Offset adjustment for voltage follower.

Figure 3.13 shows the circuit for offset voltage adjustment for voltage follower circuit

which uses a similar concept as a non-inverting amplifier circuit as shown in Figure 3.11.

However, in offset adjustment circuit, the ground level is virtually created by using

potentiometer. The adjustment of potentiometer changes the reference level to the output

voltage. To understand this circuit, the calculations will be discussed for three different cases.

That is, when potentiometer 2R is adjusted to 0%, 50%, and 100%. The output of any of

three cases is given by:

( )out in adjV V Gain V= × + , (3.7)

where: 1 2adjV V V≤ ≤ .

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In order to clarify, let 1V be equal to Vcc+ and 2V be equal to Vcc− . Therefore, at

0%, 50% and 100% of 2R , the values of adjV are equal to Vcc+ , 0 or ground level, and

Vcc− respectively. Note that at 0%, 50% and 100% of 2R , the values of 2R are equal to

zero, 2 / 2R , and 2R ohms respectively. Using equation (3.6) to calculate the gain and using to

equation (3.7) to calculate the output, the gain (with respect to ground level) and the output

voltage ( outV ) in each case can be computed as follows:

Case 1: at 0% of 2R : ( 2 0R = ohms; adjV Vcc= + volts)

3

1

1 RGainR

= + (3.8)

3

1

1out inRV V VccR

⎛ ⎞= + +⎜ ⎟

⎝ ⎠. (3.9)

Case 2: at 50% of 2R : ( 22 2RR = ohms; 0adjV = volts)

3

21

1

2

RGain RR= +

+, (3.10)

3

21

1

2

out inRV V RR

⎛ ⎞⎜ ⎟

= +⎜ ⎟⎜ ⎟+⎝ ⎠

. (3.11)

Case 3: at 100% of 2R : 2 2R R= ohms; adjV Vcc= − volts)

3

1 2

1 RGainR R

= ++

, (3.12)

3

1 2

1out inRV V Vcc

R R⎛ ⎞

= + −⎜ ⎟+⎝ ⎠. (3.13)

From above equations, the second case does not have an additional voltage (offset)

added to the output. That means the output of the circuit is referenced by a virtual ground

level. While in case 1 and 3, there are additional voltages added to the output level either

positive or negative. As a result, the offset of the output voltage can be adjusted by using this

fundamental concept.

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In our EMG amplifier device, the same chip as introduced in section 3.4.1 (Amplifier)

was used. The chip is a dual op-amp; one op-amp was used for amplifier circuit, while

another op-amp was used for bias adjustment circuit. Therefore, each channel, it needs only

one chip.

Figure 3.14: Bias adjustment circuit.

Figure 3.14 illustrates the circuit of Bias adjustment for one channel. The gain in each

case (0%, 50% and 100% of 2R ) can be computed by using the equation (3.8), equation

(3.10) and equation (3.12) where 3R is one kiloohm and 1R is ten kiloohms. As the result, the

gain in each case is equal to 1.1, 1.029, and 1.017 respectively, which is approximately equal

to one. Therefore this bias adjustment circuit approximately have no gain. The output of this

circuit is roughly the summing of input ( ( )out AmpV ) and the voltage range of +9 and -9 volts.

To adjust the reference level to positive value (up), add positive voltage to the input (case 1)

as shown in equation (3.9). To adjust the reference level to negative value (down), a negative

voltage to the input (case2) should be added as shown in equation (3.13). By using equation

(3.7), final output can be determined by:

_ ( ) 9out final out AmpV V V= ± .

The protection resistor 39R was placed into the circuit to protect a short circuit in case

when the output of the circuit ( outV ) is accidentally short-circuited to the ground level. If this

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situation happened without 39R , the circuit would be shorted. However with 39R the circuit is

protected as 39R serves as a load resistor for the circuit. Usually the protection resistor has

low value to allow electrical current to flow easily. For this reason, the value of 22 ohms is

selected for 39R .

The whole circuit diagram of the EMG amplifier shown in Appendix A. The EMG

amplifier device draws the current about 83 milliamps. The 9-volt alkaline battery has a

capacity of 500 milliamps hour [10]. Therefore, this EMG Amplifier device can be operated

continuously for approximately six hours.

3.5 A/D CONVERTER AND USER INTERFACE To record the EMG signals to a computer, a conversion of analog signal to digital

signal is needed. The analog to digital converter (A/D) is a device that converts an analog

signal to a digital signal. There are lots of A/D interface cards available in the market. The

difference of those interface cards is determined by the clock speed, the resolution bit rate,

the number of analog input/output channels, or by the software that comes with it.

The specific A/D interface card that is used in this project is the NI 6220 M-Series

Multifunction DAQ from National Instrument Company [36]. Some specifications of NI-

6220 M-Series are available in Table 3.1.

The NI 6220 DAQ card has sixteen bits resolution of analog input, and a clock speed

from eight hertz up to one megahertz, which is fast enough to capture the EMG signal for our

experiments. This A/D interface card does not provide analog output. Therefore, some

characteristics are skipped in Table 3.1. For more information about NI 6220, please see

Appendix B.

The output from EMG amplifier is connected to the analog input port of A/D

interface card. When conducting grasping experiment, the EMG signals will be captured by

using our implemented program called ‘EMG Capture’ or ‘EMGC’. The EMGC program has

an interface with the supplied driver of the A/D interface card. The EMGC program will

command the A/D converter card to read the EMG signals. The EMG data can be saved in a

text data file by using the EMGC program. These data can be analyzed for the future task

36

such as performing a feature extraction and a pattern classification process. The explanation

of usage and implementation of the EMGC program are explained in Chapter 4.

Table 3.1: NI 6220 DAQ Specification

Bus PCI

Analog Inputs 16

Analog Input Resolution (bits) 16

Analog Outputs -

Output Resolutions (bits) -

Max Output Rate (kS/s) -

Output Range (V) -

Digital I/O 24

Correlated (Clocked) DIO 8 Hz, up to 1 MHz

3.6 FINAL PRODUCT ASSEMBLY After finished design and testing of the EMG amplifier on a breadboard, next step is

to make a usable product. All the circuits implemented in the breadboard are transferred to a

printed-circuit board or PCB. All the electronics components such as resistors and capacitors,

and ICs are soldered to the PCB. Then, the PCB is covered with plastic enclosure to protect

the circuit from external environment. The only thing left to be seen are the control buttons,

input connectors and output connectors, which are standard electronic components.

Nowadays people use electronic design software for making a PCB. Most of the

electronic design software provides two modules. The first module is used to draw a

schematic. The second module is used to design a PCB. The schematic diagram is an

electronic circuit that represents each electronic component by using standard electronic

symbols. To make a PCB by using electronic design software, a schematic diagram has to be

drawn first. Once the schematic diagram is finished, the software creates a circuit connection

database. The database contains all connections of the electronic design. The database is

often called ‘netlist’. The netlist is then imported to the PCB design module. In this phase,

the software is deciding where to put all the components and draw a copper trace to connect

them together. In our case, the EMG amplifier schematic was designed by using software

37

called ‘Multisim8’ from Electronics Workbench Company [16]. The PCB was designed by

using software called ‘Ultiboard7’ from the same company [17].

Most of the electronic design software have ability to export the PCB design file into

a machine file. The PCB cutting machine uses machine file to drill holes for the components,

to cut the PCB into shape, to layout the copper trace, or to print text on PCB. The PCB

manufacturers usually accept the machine file for producing PCB.

Figure 3.15: The PCB of the EMG amplifier.

Figure 3.15 shows the finished PCB of the EMG amplifier as produced by the

manufacturer. The PCB is designed to accept two nine-volt batteries on each side. Since the

device will be placed under the forearm, placing batteries on each side distributes the weight

balance. The dimension and the layout plastic enclosure can be found in Appendix B [39].

This PCB is a double layer PCB. A double layer defines a top copper layer and a

bottom copper layer. A ‘through hole’ is used to connect trace between two layers. The text

and component layout printed on the PCB is called ‘silk screen’ layer. The Appendix A

shows layout of each layer of our EMG amplifier.

Figure 3.16 shows the EMG amplifier after soldering all components into the PCB.

There are two 9-volt batteries placed on each side. After soldering all components, PCB is

required to be cleaned to remove residue flux from pre-soldering process. Usually non

conductive substance spray such as a contact cleaner is used to clean the PCB after soldering.

38

Figure 3.16: The EMG amplifier after soldering.

Our EMG amplifier has four input channels and four output channels. The RJ-45

connector is selected to be the output connector. The RJ-45 has eight conductors. Four

conductors are used for output signals and other four conductors are used for ground signals

for each output channel. The EMG amplifier will be connected to the analog inputs of the

A/D interface card, which has 68 pins. The A/D interface card provides 68 pins for its

features such as digital inputs and outputs, and analog inputs. However, our EMG amplifier

is using only four analog input channels of the A/D interface card which is only eight pins

(four channels and ground each). The pin connections of the EMG amplifier and the A/D

interface card are shown in Table 3.2. More detailed information about the pin layout of the

A/D interface card is shown in Appendix B.

3.7 EMG AMPLIFIER CALIBRATION PROCEDURE Similar to any electronic measurement device, our EMG amplifier device needs to be

calibrated for accuracy. Calibration is performed to correct the value of each output reading

from the EMG amplifier by comparing it to the input value.

Figure 3.17 demonstrates the calibration procedure for our EMG amplifier. Since

there are four input channels of the EMG amplifier and each channel is operated separately,

each channel requires a separate calibration. The calibration procedure requires a function

generator. The function generator is used to generate the input signal for the EMG amplifier.

In our case, the sinusoidal wave with amplitude of 100 millivolts at frequency of 50 hertz is

39

Table 3.2: EMG Amplifier and A/D Interface Card Pin Connection

EMG Amplifier A/D Interface Card

Description Pin Number Pin Number Description

Vout CH1 1 68 AI 0

Gnd 2 67 Gnd

Vout CH2 3 33 AI 1

Gnd 4 32 Gnd

Vout CH3 5 65 AI 2

Gnd 6 64 Gnd

Vout CH4 7 30 AI 3

Gnd 8 29 Gnd

used as the calibration signal. This input signal is then circuitry reduced 1,000 times via

voltage a divider circuit to produce 100 µV input signal. The device of the attenuation is

determined by the fact that our EMG amplifier has a fixed gain of 1,000 at the preamplifier.

The output signal from the EMG amplifier should be equal to the input calibration

signal (100 millivolts without any offset and gain). The calibration begins by adjusting the

bias followed by adjusting the gain. In case if an arbitrary gain is needed, the amplitude of

the output signal should be equal to the desired gain value multiplied by 100 millivolts. For

instance, if the gain two is needed, the amplitude of the output signal should be equal to 200

millivolts.

The signal amplitude of 100 millivolts is used to simulate an EMG signal. The

frequency of 50 hertz is selected as the signal input for a calibration because the ambient

noise frequency occurs primarily within the range 50 hertz or 60 hertz. Such input signal is

easily contaminated by noise. If the EMG amplifier works satisfactory at this signal

condition, it is expected that it will work satisfactory at other less vulnerable signal

conditions.

As mentioned at the beginning, each channel has its own gain adjustment and bias

adjustment control. Therefore the calibration has to be performed for each channel, one

channel at a time. For an accurate measurement, each channel should have the same gain.

40

The output signal of the calibrated device should have no offset. The ground level of the

output signal should be at zero voltage.

Figure 3.17: Calibration procedure.