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Part Number: THS4531DGKEVM
Other Parts Discussed in Thread: THS4531, THS4561, OPA1637

Dear TI Support Team,

I work as an audio engineer for the Audio company Bang & Olufsen. I am evaluating the EVK of THS4531 at the moment. We plan to use this fully differential amp as a mic preamp that drives a differential ADC Input. The ADC's internal PGA gain is too noisy for our needs, especially at higher gains.

According to datasheet, the THS4531 provides a noise density of 10nV/sqrt(Hz) which corresponds to -117dBV (20Hz to 20kHz) at a gain of 1 (using 2k for Rf and Rg). However when I measure differential in to differential out with an Audio Precision, I can only measure -110dBV (A-weighted) at the THS4531 output. I use 50cm long balanced XLR cables and a source impedance of 40Ohm. The supply is 3.2V taken from a battery, single supply. I removed everything expect for the IC and Rf&Rg from the input and output path. So the noise-floor is 7dB higher as the spec in the datasheet. Do you have an idea what is going wrong here ?

When I change Rg to 470Ohm this yields a gain of 12dB. Here I measure -103.5dBV A-weighted at the output, which corresponds to -115.5dB input referred noise. For our purpose we would need an output noise of -110dB @ 12dB gain. Would this be possible with this amp?

Best Regards,