This document discusses using distortion shaping techniques to equalize the THD performance of ADCs between different automatic test equipment (ATEs). It presents a method of generating test signals with an arbitrary waveform generator (AWG) that shifts harmonic distortion components closer to the Nyquist frequency to improve the AWG's harmonic performance and allow more accurate testing of ADC THD specifications. Test results show the proposed method significantly reduces harmonic distortion in the AWG output and allows different testers to achieve identical ADC test results, resolving issues caused by performance differences between ATEs. The technique is especially effective for higher resolution ADCs where noise floors are lower.

1. Confidential © ams AG
Using Distortion Shaping Technique
to Equalize ADC THD Performance
Between ATEs
IMSTW 16
Peter Sarson CMgr MCMI SMIEEE
Full Service Foundry
4th July 2016
Prof Hauro Kobayashi
Gunma University

2. Confidential © ams AG
Page 2
Introduction
• During development of any test program test engineers sometimes face the
issue that the tester isn’t as good as the chip
• This causes issues when device specs need testing and the limit cannot be
reached
• Kobayashi will show some mathematics: harmonic distortion contents can be
shifted up close to Nyquist frequency.
• Peter will show how the harmonic performance of an ATE’s AWG can be
improved using this approach
• Peter will show how this approach improved the result and reproducibility of a
12-bit ADC testing
• Peter will show how this approach improved the result of a 16-bit ADC testing
that was limited by the tester performance

3. Confidential © ams AG
Page 3
Research Objective
Low distortion signal generation with AWG for low cost and high quality testing of ADC
Investigation of phase switching signal generation method with experiment at ATE
environment
• No need for AWG nonlinearity identification
• Only DSP program change

4. Confidential © ams AG
Page 4
ADC test signal Test result
ADC Testing with Sine Signal
Freq.
ADC HD3
Freq.
“No Go”
“Go”
Freq.
Reference level
Allowable
fundamental
Freq.
Test result
fundamental
AWG ADC
𝑍 = 𝑏1 𝑌 + 𝑏3 𝑌3
fin 3fin
fin 3fin
fin 3fin
fin

5. Confidential © ams AG
Page 5
ADC test signal Test result
Problem of Conventional Method
AWG HD3+ADC HD3.
Freq.
fundamental
Freq.
Test result
AWG ADC
𝑍 = 𝑏1 𝑌 + 𝑏3 𝑌3
fundamental
fin 3fin
fin
Test signal generation
Y=a1Din + a3Din
3
3fin
AWG HD3
HD3
ADC HD3 cannot be measured accurately
Low quality test

6. Confidential © ams AG
Page 6
ADC test signal Test result
Research Goal
Only ADC HD3
Freq.
fundamental
Freq.
Test result
AWG ADC
𝑍 = 𝑏1 𝑌 + 𝑏3 𝑌3
fundamental
fin 3fin
fin
Test signal generation
Y=a1Din + a3Din
3
3fin
AWG HD3 reduction
HD3
High quality testOnly program change

7. Confidential © ams AG
Page 7
Conventional Signal Generation with AWG
AWG
DAC
CLK
DSP
X XXX
Din
Din
AWG sampling frequency: fs(AWG) = 1/Ts
A
𝐗= Acos(2πf_in nT_s)
0 0.1 0.2 0.3 0.4 0.5
-300
-200
-100
0
Power[dB]
Normalized frequency f/fs
𝐟𝐢𝐧
𝐇𝐃𝟑

8. Confidential © ams AG
Page 8
Proposed Signal Generation with AWG
AWG
DAC
CLK
DSP
X0 X1X0X1
Din
Din
AWG sampling frequency: fs(AWG) = 1/Ts(AWG)
𝐗 𝟎= 1.15Acos(2πf_in nT_s−π/6)
𝐗 𝟏=1.15Acos(2πfin nTs+𝜋/6)
0 0.1 0.2 0.3 0.4 0.5
-300
-200
-100
0
𝐟𝐢𝐧
𝐟𝒔/2−𝟑𝐟in
𝐟𝒔/2−𝐟in
Power[dB]
Normalized frequency f/fs
Θ = π/3𝐓𝐬(𝐀𝐖𝐆)
X0
𝐗 𝟏

9. Confidential © ams AG
Page 9
Principle of 3rd Harmonics Cancellation
Two waves with phase difference π are cancelled
ConventionalPhaseSwitching
Θ = π/3 3Θ = π
3rdordernon-linearsystem
Phaserotationbyx3
fundamental: fin 3rd harmonics: 3 fin

10. Confidential © ams AG
Page 10
X0=Asin(2πfint+π/4) X1=Asin(2πfint-π/4)
HD2 Cancellation
2-way interleave cancels HD2

11. Confidential © ams AG
Page 11
X0=Asin(2πfint-π/4-π/6)
X2=Asin(2πfint+π/4-π/6)
X1=Asin(2πfint-π/4+π/6)
X3=Asin(2πfint+π/4+π/6)
HD2, HD3 Cancellation
4-way interleave cancels HD2 & HD3

12. Confidential © ams AG
Page 12
Be Careful
If you use this technique without filtering the generated high-frequency spurious
Harmonic contents of the ADC under test will be under-estimated.
These spurious components need filtering out
to get back normal sinewave without harmonics.
More detailed later.

13. Confidential © ams AG
Page 13
How to Do in Software
2-signal interleaved
Element
1
Element
2
Element
3
Element
4
Element
5
Element
6
Element
7
Element
8
Array 1 1 2 3 4 5 6 7 8
Array 2 9 10 11 12 13 14 15 16
Supressed 1 10 3 12 5 14 7 16

14. Confidential © ams AG
Page 14
How to Do in Software
4-signal interleaved
Element
1
Element
2
Element
3
Element
4
Element
5
Element
6
Element
7
Element
8
Array 1 1 2 3 4 5 6 7 8
Array 2 9 10 11 12 13 14 15 16
Array 3 17 18 19 20 21 22 23 24
Array 4 25 26 27 28 29 30 31 32
Supressed 1 10 19 28 5 14 23 32

15. Confidential © ams AG
Page 15
Test Setup for Measuring Tester Performance
AWG
MEMORY
AWG FILTER
50MHz
DIGITIZER
FFT
Tester Result
Loopback

16. Confidential © ams AG
Page 16
Tester 1 3rd Harmonic Reduction
Measured spectrum level improvement of reference vs suppressed 3rd harmonic for AWG.

17. Confidential © ams AG
Page 17
Tester 1: 3rd and 5th Harmonic Reduction
Measured spectrum levelimprovement of reference vs suppressed 3rd & 5th harmonicsforAWG.

18. Confidential © ams AG
Page 18
Harmonic Reduction
Measured Harmonic Performance Improvement for AWG Using the Proposed Method
Harmonic Improvement
2nd
6 dB
3rd
16 dB
4th
18 dB
5th
23 dB

19. Confidential © ams AG
Page 19
ADC Performance Test Setup
AWG
MEMORY
AWG FILTER
50MHz
DUT
ADC
DIGITIZER
FFT
Tester Result

20. Confidential © ams AG
Page 20
12-bit ADC Results Tester 1
signal Thd std units
ref 89,7 2,5 dB
2nd_red 89,8 2,1 dB
3rd_red 89,3 2 dB
4th_red 89,5 2,1 dB
5th_red 89,2 2,6 dB
2nd,4th_red 91 2,4 dB
3rd,5th_red 89,7 2,5 dB

21. Confidential © ams AG
Page 21
12-bit ADC Results Tester 1
signal Thd std units
ref 89,7 2,5 dB
2nd_red 89,8 2,1 dB
3rd_red 89,3 2 dB
4th_red 89,5 2,1 dB
5th_red 89,2 2,6 dB
2nd,4th_red 91 2,4 dB
3rd,5th_red 89,7 2,5 dB

22. Confidential © ams AG
Page 22
12-bit ADC Results Tester 1
signal Thd std units
ref 89,7 2,5 dB
2nd_red 89,8 2,1 dB
3rd_red 89,3 2 dB
4th_red 89,5 2,1 dB
5th_red 89,2 2,6 dB
2nd,4th_red 91 2,4 dB
3rd,5th_red 89,7 2,5 dB

23. Confidential © ams AG
Page 23
12-bit ADC Harmonic Suppressed Result
2nd and 4th suppressed delta and overall THD Improvement
THd ref THd supp
Tester 2 88,7dB 91dB
Tester 1 89,6dB 91dB
Without suppression:
• Different measured values of THD performance
With Suppression:
• Exactly the same result between Testers 1 & 2

24. Confidential © ams AG
Page 24
Site to Site Comparison
No difference between sites with the proposed method!
12-bit ADC testing case
Tester 1 Site 1 Site 2 delta
Red 91dB 91,1dB 0,1dB
ref 89,6dB 89,6dB 0dB

25. Confidential © ams AG
Page 25
3rd, 5th and 7th Suppressed

26. Confidential © ams AG
Page 26
How 8-signal interleaved looks like
2^3 = 8

27. Confidential © ams AG
Page 27
3-Harmonics (3rd , 5th & 7th Harmonics) Suppression
Using 3-harmonics suppression algorithm, effect of the sidebands would be SB= fs/8
– 7*fin
Maximum sampling freq. (fs) of AWGHSB : 250MHz Signal freq. (fin): 1MHz
Filter with 24MHz cutoff is needed
Not available on AWGHSB
Need a load-board filter.

28. Confidential © ams AG
Page 28
OVERSAMPLE = 1
FS: 259.2000 kHz
THD = -89.83dB
Another Example – Tester 1 (Conventional)
Obelic 16-bit ADC – no suppression, direct conversion

29. Confidential © ams AG
Page 29
OVERSAMPLE = 1
FS: 259.2000 kHz
THD = -89.83dB
Another Example – MX01 (Conventional)
16-bit ADC – no suppression, direct conversion

30. Confidential © ams AG
Page 30
OVERSAMPLE = 600
FS: 155.5200 MHz
THD = -115.09dB
MUCH BETTER
Obelic 16-bit ADC – 2nd Harmonic Suppressed (Proposed)
Oversample 600 times – Tester 1

31. Confidential © ams AG
Page 31
OVERSAMPLE = 600
FS: 155.5200 MHz
THD = -113.87dB
Same as Tester 1
16bit ADC – 2nd Harmonic Suppressed (Proposed)
Oversample 600 times – Tester 2

32. Confidential © ams AG
Page 32
Performance summary
Using a 12-bit ADC, testing performance can be improved but you are limited to the
noise performance of the ADC itself
• If spec limits are an issue, the technique can help squeeze out some margin
Using a 16-bit ADC, performance can be significantly improved (say, by 25 dB) as the
noise floor is much lower due to the number of bits.
Effectiveness of the distortion-shaping technique was verified with ATE
environment.

33. Confidential © ams AG
Thank you
Please visit our website www.ams.com

34. Confidential © ams AG
Page 34
Appendix
Proposed signal generation method
• Distortion components close to signal band are reduced
• Distortion components far from signal band may appear.
Similar to but different from noise-shaping
Distortion-shaping
Normalized frequency f/fs