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I am using the THS4531AIRUNR to drive the differential inputs of an ADS1251 analogue to digital converter in a multi channel EEG amplifier as shown in the Partial circuit below. R24 is driven by the output of an instrumentation amplifier and the THS4531A converts the this single ended signal to a differential signal suitable for driving the ADS1251.
I have noticed that a couple of the channels displayed what looks like burst noise, more commonly called popcorn noise when the input to the instrumentation amplifier was shorted. This can be seen in channel 35 on the screen shot below - the sudden switching between two levels is a clear indication of popcorn noise. Channel 36 shows a trace without popcorn noise. Replacing the instrumentation amplifier did not remove the popcorn noise but replacing the THS4531A amplifier did.
The data was captured at 20000 samples per second then filtered using a second order IIR low pass filter with a -3dB frequency of 70Hz - a typical diagnostic EEG bandwidth.
My question is whether the THS4531AIRUNR is known to have popcorn / burst noise or is there some other explanation?
Hi Michael,
Attached is the CH35 trace scaled with respect to the input of the THS4531A. Vertical scale is microvolts, horizontal in seconds. Data acquired at 20000 samples per second then low pass filtered with 2nd order Butterworth IIR with a -3dB point of 70Hz.
I expected the noise to be about 20uVpk-pk (if no popcorn) - with the THS4531A contributing 4uVpk-pk, the ADS1251 14uVpk-pk and the preceding instrumentation amplifier gain (G=11.4) stage around 12uVpk-pk. For comparison, CH36, which does not exhibit popcorn noise, is shown below.
Although the datasheet suggests 2kilohm feedback resistors for minimum distortion I chose 4.7kilohm to reduce power consumption as ultra low distortion is not required for diagnostic EEG. The ADC reference is 4.096V so it is possible for the THS4531A outputs to be each swinging greater than 4Vpk-pk. Thus consuming more power in the feedback and gain resistors than in the THS4531A.
During the noise test the preceding instrumentation amplifier inputs are switched to ground so the signal driving R24 (see previous post) should be close to ground.
