A DSP-Based Ramp Test for On-Chip High-Resolution ADC

Wei Jiang and Vishwani D. Agrawal

Auburn university

Motivation

• Testing analog-to-digital converter (ADC)– Linear ramp to cover full range of ADC– Slow slope for static testing– Histogram-based non-linearity error

• Proposed DSP-based ramp test– Characterizing ADC using linear function– Estimating coefficients of the function

ICIT'11 2

Non-linearity Error

• Least significant bit (LSB)

• Signal values at lower and upper edges of each codes

• Non-linearity errors– Differential non-

linearity (DNL)– Integral non-linearity

(INL)

ICIT'11 3

kkk

k

ikk

kkk

kkk

kk

kk

DNLINL

LSBDNL

LSB

LSB

2

ˆˆ2

ˆˆ2

ˆˆ

5.0ˆ

5.0ˆ

1

1

2

1

1

Typical ADC Testing Architecture

ICIT'11 4

TPG

ORA

TEST CONTROL

MUX

MUX

DAC

ADC

MIXED SIGNAL

MUX

ANALOG SYSTEM

ANALOG SYSTEM

MUX

BIST Results

Digital System

Input and Output

Analog System

Input and

Output

Analog Loopbacks

DSP

DIGITAL SYSTEM

Digital loopback

Digital input

Loopback controls

Test pattern control

Response control

Digital outputAnalog signals

Analog loopback

Analog system loopback

Analog output

Analog input

under-test

under-test

under-test

* F. F. Dai and C. E. Stroud, “Analog and Mixed-Signal Test Architectures,” Chapter 15, p. 722 in System-on-Chip Test Architectures: Nanometer Design for Testability, Morgan Kaufmann, 2008.

Ramp Test Structure for ADC

ICIT'11 5

Similar to structures for histogram approach

Ramp Test

• Histogram Test for high-resolution ADC– A large amount of code to be tested– Multiple samples for each code– Very low-slope ramp testing signals required

• Comparable to thermal noise

• Proposed Approach– Linear function to characterize ADC– Coefficients of the function are easy to calculate– Only part of codes measured; speed up testing

time

ICIT'11 6

Linear Function

• Linear ramp function– T is the sampling

time

• Measured output– K is the maximum

code

• Reconstructed ramp function using measured samples

ICIT'11 7

Kk

KkkfM

K

kMNADCKk

12

1..1

00

0

bkTakf

qeLSBkMkf ˆ

Division of Measurements

• Divided full-range of ADC codes into two equal-size sections

• Sum up measurements of each section

• Lower bound M(0) and upper bound M(K) are discarded because of possible out-of-range measurements

ICIT'11 8

KbaTKKLSB

kfLSB

kMs

KbaTKKLSB

kfLSB

kMs

K

Kk

K

Kk

K

k

K

k

2

122

8

11

1

2

12

8

11

1

1

2/

1

2/0

2/

1

2/

10

Calculation of Coefficients

• Two syndromes obtained from sums

• LSB, K and T are all known parameters

• Estimated coefficients calculated from syndromes

ICIT'11 9

2

421

1

2

41

1

3

24

11

011

0

011

010

KK

SSKS

LSBb

TKK

S

LSBa

baTKLSB

ssS

aTKKLSB

ssS

BIST Steps - measurements

• Reset ramp testing signal generator• Detect first non-zero ADC output

(lower-bound of samples)• Measure all subsequent samples• Stop at the maximum ADC output

(upper-bound of samples)• DSP collects all valid measurements

and start to processing data

ICIT'11 10

BIST Steps – processing data• Divide measured samples into two

equal-size parts• Accumulate measurements of each part

to obtain two sums• Calculate two syndromes from two sums• Calculate two estimated coefficients of

the linear ramp function• (Optional) Compare each measured data

to estimated one from ramp function

ICIT'11 11

Design of Ramp Signal Generator

ICIT'11 12

Switch for resetting ramp

I I

I/30

ΔV+Vth ΔV

Range: 0 v~ Vdd-ΔV

Simulation Results

ICIT'11 13

Simulation Results

ICIT'11 14

Other considerations

• Minimal number of samples– More samples, less quantization noise, more

accurate estimation– Not all codes need to be sampled in order to

reduce testing time– At least 2N-2 samples are found necessary in

practice

• The same idea may be used with low-frequency sinusoidal testing signals instead of ramp signal– More overhead and complexities with

sinusoidal generator

ICIT'11 15

Conclusion

• Proposed Approach– For high-resolution ADC– Less samples required comparing to

histogram approach– Simple algorithm to calculate

coefficients and make estimation

16ICIT'11

THANK YOU

ICIT'11 17