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Download 2188 Input Bias Current Commutation
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Characterizing Bias Current Spikes Device Under Test Input Current Wideband TIA A wideband transimpedance amplifier was used to directly view the input current of several chopper amplifiers TI Information – Selective Disclosure 1 Characterizing Bias Current Spikes VOUT RF I B (U 1) I B (U 2) I N (U 1) I N (U 2) I Spike Bias Current (IB) Current Noise (IN) OPA657 OPA2188 2 pA 160 pA 1.3 fA/rtHz 7 fA/rtHz Bias current spikes should be apparent above other noise sources TI Information – Selective Disclosure 2 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 3 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 4 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 TI Information – Selective Disclosure 5 AD8639 1.7 uA! This is not unique to TI auto-zero topologies! TI Information – Selective Disclosure 6 Noise Feed-Through Without proper design considerations noise from the input current spikes can appear in the output TI Information – Selective Disclosure 7 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 8 Contribution to Output Noise Current spikes on the inverting input are coupled to the load by the feedback network V1 IG 2 ( RG || RF RLOAD ) VOUT V1 RLOAD RF RLOAD Output noise is dependant upon: – Input current spike magnitude – Feedback network impedance (RF and RG) – Load Impedance (RLOAD) TI Information – Selective Disclosure 9 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 Noise(V / Hz ) AV VRMS K N RBW VRMS: Output voltage AV: Gain of secondary amplifier KN: Brickwall correction factor (1.056) RBW: Resolution Bandwidth TI Information – Selective Disclosure 10 Gain Effects on Total Noise At high gains chopper noise is not a dominant noise contributor TI Information – Selective Disclosure 11 Load Impedance Effects High load impedances can exacerbate chopper noise from input current spikes TI Information – Selective Disclosure 12 Feedback Network Impedance Large feedback resistor values will also worsen the output noise TI Information – Selective Disclosure 13 Output Filtering fC 1 2 ROUT COUT 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 TI Information – Selective Disclosure 14 Output Filtering fC ( SPIKE ) 2 ( ROUT 1 RF ) COUT • 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 15 Output Filtering fC fC ( SPIKE ) 1 2 ROUT COUT 2 ( ROUT TI Information – Selective Disclosure 1 159kHz 2 *100 *10nF 1 1 1.576kHz RF ) COUT 2 (100 10k) 10nF 16 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 17 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 TI Information – Selective Disclosure 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 18 Comparison to Non-Autozero Amplifiers The noise level of a filtered chopper amplifier is on-par with non-chopper topologies TI Information – Selective Disclosure 19 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 TI Information – Selective Disclosure 20