1TI Information – Selective Disclosure

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

2TI Information – Selective Disclosure

Characterizing Bias Current Spikes

SpikeUNUNUBUBFOUT IIIIIRV )2()1()2()1(

OPA657 OPA2188

Bias Current (IB) 2 pA 160 pA

Current Noise (IN) 1.3 fA/rtHz 7 fA/rtHz

Bias current spikes should be apparent above other noise sources

3TI Information – Selective Disclosure

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

4TI Information – Selective Disclosure

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

5TI Information – Selective Disclosure

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

6TI Information – Selective Disclosure

AD8639

This is not unique to TI auto-zero topologies!

1.7 uA!

7TI Information – Selective Disclosure

Noise Feed-Through

Without proper design considerations noise from the input current spikes can appear in the output

8TI Information – Selective Disclosure

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

9TI Information – Selective Disclosure

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

)||(21 LOADFGG RRRIV

LOADF

LOADOUT RR

RVV

1

10TI Information – Selective Disclosure

Noise Measurement

RBWKA

VNoise

NV

RMSHzV

)/(

• 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

11TI Information – Selective Disclosure

Gain Effects on Total Noise

At high gains chopper noise is not a dominant noise contributor

12TI Information – Selective Disclosure

Load Impedance Effects

High load impedances can exacerbate chopper noise from input current spikes

13TI Information – Selective Disclosure

Feedback Network Impedance

Large feedback resistor values will also worsen the output noise

14TI Information – Selective Disclosure

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

OUTOUTC CRf

21

15TI Information – Selective Disclosure

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

OUTFOUTSPIKEC CRR

f

)(2

1)(

16TI Information – Selective Disclosure

Output Filtering

kHznFCR

fOUTOUT

C 15910*100*2

1

2

1

kHznFkCRR

fOUTFOUT

SPIKEC 576.110)10100(2

1

)(2

1)(

17TI Information – Selective Disclosure

Output Filtering

• Harmonics due to input current spikes are completely eliminated

• Autozero noise at chopping frequency is within the opamp bandwidth

18TI Information – Selective Disclosure

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

19TI Information – Selective Disclosure

Comparison to Non-Autozero Amplifiers

The noise level of a filtered chopper amplifier is on-par with non-chopper topologies

20TI Information – Selective Disclosure

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