Characterizing Bias Current Spikes

1. Characterizing Bias Current Spikes A wideband transimpedance amplifier was used to directly view the input current of several chopper amplifiers Device Under Test Input Current Wideband TIA

2. Characterizing Bias Current Spikes Bias current spikes should be apparent above other noise sources

3. Input Bias Spike Measurement Equipment • A shielded enclosure was used to mitigate extrinsic noise • Direct connection to the oscilloscope via coax cable Spikes were viewed on a 500 MHz oscilloscope • 50 Ohm input impedance • DC coupling (maximize bandwidth) • Averaging used to remove random noise

4. OPA2188 • Spikes repeat at 2x the chopper clock frequency • Larger spike is due to the input commutation • Smaller spike is from the synchronous notch filter on the output • Largest spike peaks at 850nA • Total duration is ~24nS

5. OPA2333 Commutation frequency is much lower • Input clock spikes are now of similar magnitude to notch filter spikes Different input topology • Transmission gates reduce input current spikes • 70 nA peak • 216 nS duration

6. AD8639 This is not unique to TI auto-zero topologies! 1.7 uA!

7. Noise Feed-Through Without proper design considerations noise from the input current spikes can appear in the output

8. Equivalent Schematic Input current spikes can be viewed as current sources on the inputs Input current spikes are outside the opamp bandwidth • The opamp can be removed to simplify analysis • Current spikes on the non-inverting input are not amplified

9. Contribution to Output Noise Current spikes on the inverting input are coupled to the load by the feedback network Output noise is dependant upon: • Input current spike magnitude • Feedback network impedance (RF and RG) • Load Impedance (RLOAD)

10. Noise Measurement • Output noise was amplified and viewed on a spectrum analyzer • OPA2211 with a gain of 11 • HP3588 (10Hz to 150MHz) • Data collected using LabView • Shielded enclosure and cables VRMS: Output voltage AV: Gain of secondary amplifier KN: Brickwall correction factor (1.056) RBW: Resolution Bandwidth

11. Gain Effects on Total Noise At high gains chopper noise is not a dominant noise contributor

12. Load Impedance Effects High load impedances can exacerbate chopper noise from input current spikes

13. Feedback Network Impedance Large feedback resistor values will also worsen the output noise

14. Output Filtering Adding an RC output filter can mitigate noise seen by high impedance loads • COUT chosen to have an impedance much less than RLOAD at 2x chopping frequency • ROUT chosen to maintain opamp stability with the chosen COUT

15. Output Filtering • The corner frequency for the input current spike is actually much lower • The filter now includes the feedback resistance RF • Filter corner frequency can be chosen to remove noise without affecting desired signal

16. Output Filtering

17. Output Filtering • Harmonics due to input current spikes are completely eliminated • Autozero noise at chopping frequency is within the opamp bandwidth

18. Output Filtering OPA2188 Without Filtering • Gain: 101, RF:10k, RG:100 Ohm • Oscilloscope 1MOhm input impedance is the load • Input current spikes are visible above other noise sources OPA2188 With Filtering • Gain: 101 RF: 10k, RG: 100 Ohm • Oscilloscope 1MOhm input impedance is the load • Triggering oscilloscope becomes difficult due to low noise

19. Comparison to Non-Autozero Amplifiers The noise level of a filtered chopper amplifier is on-par with non-chopper topologies

20. Minimizing Chopper Noise Effects • Input current spikes are not amplified by the part • Spikes on the inverting input will be coupled to the load by the feedback network • Minimize feedback resistance values • Reduces the voltage produced by current spikes • Standard design practice for low-noise, low-drift circuits • Load impedance directly contributes to the magnitude of voltage produced by the spike • An RC filter is an extremely effective way to reduce output noise • Corner frequency can be placed outside of the signal bandwidth • Noise through the feedback network experiences a much greater attenuation