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

TI Information – Selective Disclosure 2 Characterizing Bias Current Spikes OPA657OPA2188 Bias Current (I B )2 pA160 pA Current Noise (I N )1.3 fA/rtHz7 fA/rtHz Bias current spikes should be apparent above other noise sources

TI Information – Selective Disclosure 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

TI Information – Selective Disclosure 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

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

TI Information – Selective Disclosure 6 AD8639 This is not unique to TI auto-zero topologies! 1.7 uA!

TI Information – Selective Disclosure 7 Noise Feed-Through Without proper design considerations noise from the input current spikes can appear in the output

TI Information – Selective Disclosure 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

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

TI Information – Selective Disclosure 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 V RMS : Output voltage AV: Gain of secondary amplifier KN: Brickwall correction factor (1.056) RBW: Resolution Bandwidth

TI Information – Selective Disclosure 11 Gain Effects on Total Noise At high gains chopper noise is not a dominant noise contributor

TI Information – Selective Disclosure 12 Load Impedance Effects High load impedances can exacerbate chopper noise from input current spikes

TI Information – Selective Disclosure 13 Feedback Network Impedance Large feedback resistor values will also worsen the output noise

TI Information – Selective Disclosure 14 Output Filtering Adding an RC output filter can mitigate noise seen by high impedance loads –C OUT chosen to have an impedance much less than R LOAD at 2x chopping frequency –R OUT chosen to maintain opamp stability with the chosen COUT

TI Information – Selective Disclosure 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

TI Information – Selective Disclosure 16 Output Filtering

TI Information – Selective Disclosure 17 Output Filtering Harmonics due to input current spikes are completely eliminated Autozero noise at chopping frequency is within the opamp bandwidth

TI Information – Selective Disclosure 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

TI Information – Selective Disclosure 19 Comparison to Non-Autozero Amplifiers The noise level of a filtered chopper amplifier is on-par with non-chopper topologies

TI Information – Selective Disclosure 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