Unit 6: DAC Testing

1
Unit 6 DAC Testing

• Intrinsic parameters test
• Gain, offset, INL, DNL, DC PSSR
• Selected-code method
• Dynamic test
• Settling time, rise/fall time, overshoot and
  undershoot, DAC to DAC Skew, glitch energy
• Tests for common DAC applications

2
Comparison of DACs and ADCs

• Digital-to-analog converter
• Code-to-voltage transfer characteristic
• One-to-one mapping

• Analog-to-digital converter
• Voltage-to-code transfer characteristic
• Many-to-one mapping

1.5
15
DAC Output Voltage
ADC Output Code
1
10
0.5
5
0
0
0
5
10
15
0
0.5
1
1.5
DAC Input Code
ADC Input Voltage
3
Intrinsic versus Transmission Parameters

• Transmission (performance) parameters
• SNR, THD, ENOB,
• Describe the quality of transmitted signals
• Dependent on the nature of the test stimulus
• Intrinsic parameters
• Absolute error, INL and DNL
• Independent to the nature of the test stimulus
• Production testing strategy is often dependent on
  specific application

4
DAC Failure Mechanisms

• Different types of DACs
• Binary-weighted architecture
• Resistive divider architecture
• Pulse-width-modulated architecture
• Pulse-density-modulated (sigma-delta)
  architecture
• Hybrids of these architectures
• Each architecture has a unique set of strengths
  and weaknesses.
• Each architectures weaknesses determine its
  likely failure mechanisms, and test methodology.

5
Transfer Curve of a DAC
Gain error
Analog output
Ideal full scale output
Actual full scale output
Actual full scale offset
Non- monotonic
INL
Device FSR
Actual output
Optimum output
Actual LSB step
Calculated LSB size
DNL actual step calculated size
Offset error
DAC Input Code
6
Gain Error

• Endpoint-referenced

• Best-fit

1.5
1.5
DAC Output Voltage
DAC Output Voltage
1
1
0.5
0.5
0
0
0
5
10
15
0
5
10
15
DAC Input Code
DAC Input Code
7
Offset Error Endpoint-referenced

• Difference between the ideal and actual output
  values when the zero or null level digital input
  code is applied.
• Expressed in FS or in fractional LSB.

• Start-point-referenced

• Midpoint-referenced

1.5
0.7
DAC Output Voltage
DAC Output Voltage
1
0.2
0
0.5
-0.3
0
-0.0
0
5
10
15
-8
7
0
DAC Input Code
DAC Input Code
8
Offset Error Best-fit

• Start-point-referenced

• Midpoint-referenced

1.5
0.7
DAC Output Voltage
DAC Output Voltage
1
0.2
0
0.5
-0.3
0
-0.8
0
5
10
15
-8
7
0
DAC Input Code
DAC Input Code
9
Least Significant Bit (LSB) Step Size

• Definition The average step size of the DAC
  transfer curve
• Three Interpretative ways
• The slope of the best-fit line. (LSB Gmeas)

10
Differential Non-Linearity (DNL)

• DNL is a figure of merit that describes the
  uniformity of the LSB step sizes between DAC
  codes.
• Four basic types of DNL calculations
• Absolute
• Endpoint
• Best-fit
• Best-straight-line

1
0.5
DNL (LSBs)
0
-0.5
Based the line that gives the best answer for
INL. (most relaxed calculation)
-1
-8
8
0
-6
-4
-2
2
4
6
DAC Input Code
11
Integral Non-Linearity (INL)

• A comparison between the actual DAC curve and one
  of three lines the best-fit line, the endpoint
  line, or the ideal DAC line.
• Alternative,

where,
12
INL Best-fit versus Endpoint

• Two shapes are similar but INL values are quite
  different

Endpoint results
Best-fit results
1
1
0.5
0.5
INL (LSBs)
INL (LSBs)
0
0
-0.5
-0.5
-1
-1
-8
8
0
-6
-4
-2
2
4
6
-8
8
0
-6
-4
-2
2
4
6
DAC Input Code
DAC Input Code
13
Major Carrier Testing

• All-codes testing is time-consuming process.
• Major carrier testing
• Selected-code testing
• For binary-weighted architecture DACs
• Measurement setting
• Measure DAC output with -FS input code ? DC Based
• Measure Wn (DC based) by setting Dn to one and
  all other bits to zero. ? Need high-accuracy
  wide-range meters

14
Major Carrier Testing (cont.)

• Alternative measurement method
• Measure major carrier transition.
• Major carrier transition The voltage (or
  current) transition between the DAC codes 2k-1
  and 2k.
• Example 0001111 ? 0010000
• DC Base Measured DAC output with minus full
  scale code
• V0 W0
• V1 W1 W0
• V2 W2 (W1W0)
• V3 W3 (W2W1W0)
• ...
• Vn Wn (Wn-1Wn-2Wn-3...W0)
• Using a voltmeters sample-and-difference mode,
  giving highly accurate measurements.

15
Segmented Method

• Another selected-code testing technique
• For segmented DAC
• All-code method 212 4096 measurements
• Segmented method 2 ? 26 128 measurements

Coarse DAC LSB size 26?VLSB
DAC code bits D11-D6
6-Bit DAC
12-bit DAC output LSB size VLSB
DAC code bits D5-D0
6-Bit DAC
Fine DAC LSB size VLSB
16
Other Selected-Code Techniques

• NIST technique is based on linear algebra and
  data collected from production lots to create
  empirical models of DACs and ADCs.
• G.N. Stenbakken and T. M. Sounders, "Linear error
  modeling of analog and mixed-signal devices",
  IEEE International Test Conference, pp. 573-581,
  1991.
• Wavelet transforms to predict overall performance
  of converters.
• Y. Takahiro and M. Soma, "Dynamic testing of ADCs
  using wavelet transforms," IEEE International
  Test Conference, pp. 379-388, 1997.

17
DC PSSR
ex. 100mVpp, 20KHz sine wave
VDD
1 1
Vout Full Scale
Input code
DAC
Ex 5mVpp

• Usually, a DAC has the worst PSS performance at
  its full scale and/or minus full scale settings
• Since these settings tie the DAC output directly
  to a voltage derived from the power supply.

18
Conversion Time (Settling Time)

• Referenced to input digital signal

• Referenced to the DAC output 50 point

Error Band
Error Band
Final Voltage
Final Voltage
50 point
DAC output
DAC output
DAC Input
DAC Settling Time
DAC Settling Time

• Measured as the DAC transitions from -FS to FS
  and vice versa. (largest voltage swing)

19
Overshoot and Undershoot

• Usually, calculated from the samples collected
  during the DAC settling time test

50 Overshoot
10 Undershoot
100 Reference
10 Undershoot
50 Overshoot
20
Rise/Fall Time

• Can also be measured from the digitized waveform
  collected during a settling time test
• Usually, defined as the time between 10 - 90

Fall Time
90
90
100 Reference
10
10
Rise Time
21
DAC to DAC Skew

• The degree of mismatch between DACs outputs.
• Measured by comparing the timing of the 50 point
  of each output to the 50 point of the others

Red DAC
Green DAC
Blue DAC
50 point
Green to Blue skew
Red to Green skew
Blue to Red skew
22
Glitch Energy

• Input 0111 ? 1000 (largest major transition)
• Glitch energy the areas that fall outside the
  rated error band. (unit ps-V)

Rising edge glitch energy
Error Band
Major carrier Transition 1-LSB swing
Falling edge glitch energy
23
Transmission Parameters Tests
AWG

Waveform source memory
Anti- Imaging filter
AWG
Waveform encoding (math)
DUT

Waveform capture memory
DUT anti- imaging filter
DUT ADC
Waveform source memory
DUT anti- imaging filter
DUT DAC
Waveform analysis (math)
Digitizer

Waveform capture memory
Anti-aliasing filter
Digitizer
24
Frequency Domain Analysis
Fundamental
(a)
(b)
(c)
(d)
(e)
digitized sine wave
Vout sine wave
Amplitude(dB)
Digitizer
FFT
Harmonic
Noise
Frequency

• (a) SFDR (spurious-free dynamic rang)
• (b) SNR (Signal to noise ratio)
• (c) SNDR (signal to noise and distortion ratio)
• (d) Dynamic range
• (e) Average noise level

25
Tests for Common DAC Applications

• DC (or slow-moving) references
• AC transmission parameters are probably
  unimportant
• Intrinsic parameters tests INL, DNL,
  monotonicity, FS range, drive capability,
  settling time
• Audio reconstruction
• To reproduce digitized sound
• Test transmission parameters
• Intrinsic parameters are measured during device
  characterization
• Dynamic tests are not typically specified or
  performed

26
Tests for Common DAC Applications (cont.)

• Data modulation
• To convert digital data into an analog signal
  suitable for transmission
• Transmission parameter tests (multi-tone) are
  performed
• Intrinsic parameters are only for device
  characterization
• Some very specific parameters such as Error
  Vector Magnitude (EVM), Phase Trajectory Error
  (PTE) may be needed
• Defined in standards documents published by the
  IEEE, NIST, ...
• Test engineer is responsible for translating the
  measurement requirements into ATE-compatible tests

27
Tests for Common DAC Applications (cont.)

• Video signal generators
• To control the intensity and color of pixels in
  displays
• Two basic types of video DAC applications, RGB
  and NTSC.
• RGB DACs
• Color or intensity distortions, vertical lines,
  ghost images, etc.
• INL, DNL, settling time, DAC skew are tested.
• NTSC DACs
• The ability to produce accurate amplitude and
  phase shifts in a 3.54 MHz sine wave, while
  changing its offset.
• Transmission parameters (gain, SNR, differential
  gain, phase) are tested.