Amplifier Settling and Charge Bucket Filter Design TIPL

Amplifier Settling and Charge Bucket Filter Design TIPL 4405 -L TI Precision Labs – ADCs by Art Kay, Dale Li 1

Required/Recommended Equipment • Calculation – Determine initial values for RC charge bucket circuit and bandwidth using Analog Engineer’s Calculator • Simulation – Optimize RC charge bucket circuit using TINA SPICE – Confirm final settling error is less than half of one LSB. • Measurement – Verify THD and SNR for good and bad charge bucket filter design. – PLABS-SAR-EVM-PDK – http: //www. ti. com/tool/plabs-sar-evm-pdk – Download EVM software and purchase EVM 2

Find amplifier and RC circuit Amplifier: • 5 V, Rail-to-Rail I/O with Zero Crossover Distortion Required • Find bandwidth using Analog Engineer’s Calculator • Use parametric search to find device. • Verify model Open Loop Gain and Open Loop Output Impedance RC Charge Bucket Circuit: • Use Analog Engineer’s Calculator for initial values • Use TINA Simulation to Optimize 3

Find the Op Amp Bandwidth and RC Charge Bucket 1. Enter the information from the ADS 8860 Data Sheet. 2. Results will be used in the simulation 4

Op Amp Model: Open Loop Gain 5

Op Amp Model: Open Loop Output Impedance 6

Find the Op Amp Bandwidth and RC Charge Bucket 7

Parameter Step Rfilt 8

Connect the hardware 9

Record results as we progress through Experiment Device Samp. Rate Voffset (V) Vin (V) 1 M 1 M 500 k 2. 5 1. 8 4. 9 3. 6 ADS 8860 Data Sheet 1 2 3 4 5 OPA 320 Good filter 1 OPA 320 Bad filter OPA 333 Low Bandwidth OPA 316 Crossover Simulated Example Settling Measurements Error ½LSB=38 u. V Verror SNR THD (V) (d. B) 93 -108 28 u. V -41 m. V -91 m. V 36. 7 m. V 47 u. V 93. 3 82. 5 54. 1 86. 1 90. 3 Your Measurements SNR (d. B) THD (d. B) -108. 8 -73. 4 -55. 9 -85. 0 -102. 4 10

Start & Setup the Plabs-Power Scaling EVM Software 1. Select “Plabs-SAR-EVM” from “start>All Programs” 2. The green “HW Connected” and teal “PSI Controls” indicate good hardware communication. 11

1: OPA 320_Good Filter 1 Note RC circuit is from optimization. Error Target = 38µV Simulate Error = 28µV 12

1: OPA 320_Good Filter 1 ADS 8860 Spec. Good Filter 1 SNR (d. B) 93 93. 3 THD (d. B) -108. 8 1. Install OPA 320_Good filter 1 coupon card in socket. 3. Press “Capture” 4. Record AC performance 2. Amplitude = 4. 9 V Offset = 2. 5 V Frequency = 2 k. Hz 13

2: OPA 320 Bad Filter Optimal values: R = 24. 9Ω C = 1. 1 n. F Bad Filter Values: R = 100Ω C = 1. 1 n. F Error Target = 38µV Simulate Error = -41 m. V 14

2: OPA 320 Bad Filter ADS 8860 Bad Filter SNR (d. B) 93 82. 5 THD (d. B) -108 -73. 4 1. Install OPA 320_Bad Filter coupon card in socket. 3. Press “Capture” 4. Record AC performance 2. Amplitude = 4. 9 V Offset = 2. 5 V Frequency = 2 k. Hz 15

3: OPA 333 Low Bandwidth Required: Gain Bandwidth= 17. 8 MHz OPA 333 Bandwidth: Gain Bandwidth= 350 k. Hz Error Target = 38µV Simulate Error = -88 m. V 16

3: OPA 333 Low Bandwidth ADS 8860 Low Bandwidth SNR (d. B) 93 54. 1 THD (d. B) -108 -55. 9 1. Install OPA 333_Low Bandwidth coupon card in socket. 3. Press “Capture” 4. Record AC performance 2. Amplitude = 4. 9 V Offset = 2. 5 V Frequency = 2 k. Hz 17

4: OPA 316 Crossover, fs = 1 Msps Bandwidth Required: Gain Bandwidth= 17. 8 MHz OPA 316 Bandwidth: Gain Bandwidth= 10 MHz Error Target = 38µV Simulate Error = 35 m. V 18

4: OPA 316 Crossover, fs = 1 Msps ADS 8860 Crossover 1 Msps SNR (d. B) 93 86. 1 THD (d. B) -108 -85. 0 3. Note that the sampling rate is 1 Msps 1. Install OPA 316_Crossover coupon card in socket. 4. Press “Capture” 5. Record AC performance 2. Amplitude = 3. 6 V Offset = 1. 8 V Frequency = 2 k. Hz 19

Input signal range to avoid crossover distortion Crossover happens at 3. 8 V for OPA 316 Hands-on Experiment - Crossover Distortion 20

Changing the Sampling Rate to 500 k. Hz 21

Changing the Sampling Rate to 500 k. Hz Acquisition time (tacq) from last slide. Bandwidth Required: Gain Bandwidth= 4 MHz OPA 316 Bandwidth: Gain Bandwidth= 10 MHz 22

5: OPA 316 Crossover, fs = 500 ksps Bandwidth Required: Gain Bandwidth= 4 MHz OPA 316 Bandwidth: Gain Bandwidth= 10 MHz Error Target = 38µV Simulate Error = 47µV 23

OPA 316 Crossover, fs = 500 ksps ADS 8860 Crossover 1 Msps SNR (d. B) 93 86. 1 THD (d. B) -108 -85. 0 Crossover 500 ksps 90. 3 -102 3. Set sampling rate to 500 ksps 1. No device change needed. 4. Press “Capture” 5. Record AC performance 2. Amplitude = 3. 6 V Offset = 1. 8 V Frequency = 2 k. Hz 24

Settling can be improved 25

Measured vs Expected Results Device Samp. Rate Voffset (V) Vin (V) 1 M 1 M 500 k 2. 5 1. 8 4. 9 3. 6 ADS 8860 Data Sheet 1 2 3 4 5 OPA 320 Good filter 1 OPA 320 Bad filter OPA 333 Low Bandwidth OPA 316 Crossover Your results should show the same trend as the expected result but the specific values will differ. Simulated Example Settling Measurements Error ½LSB=38 u. V Verror SNR THD (V) (d. B) 93 -108 28 u. V -41 m. V -91 m. V 36. 7 m. V 47 u. V 93. 3 82. 5 54. 1 86. 1 90. 3 Your Measurements SNR (d. B) THD (d. B) -108. 8 -73. 4 -55. 9 -85. 0 -102. 4 26

Thanks for your time! 27