THS4551: Simple fully-differential amplifier config

Daniel Berenguer

Part Number: THS4551

Dear experts,

I'm trying to amplify a 4.2 mVDC differential signal coming from a load cell into something 30 times greater before entering the resulting voltage into a differential ADC stage. This is my first fully-differential amplifying design but I thought this should be quite straightforward. The following is my circuit design based on the THS4551 with 16-Pin VQFN package:

Real voltages measured from the circuit are marked in blue. As you can see, R127/R17 and R128/R18 are both 30k/1k, resulting in a theoretical gain of 30. However, real measurements done between AIN2 and AIN1 (differential output voltage) are -14.2mVDC so I'm barely getting a gain of 3.38.

I think I'm missing something about the THS4551 or I'm probably using the wrong design. Could anyone help me please solve this puzzle?

Thanks so much for your time.

over 7 years ago

0 Daniel Berenguer over 7 years ago

Expert 2185 points

BTW, VAA = 5VDC

0 Jacob Freet over 7 years ago

TI__Expert 8080 points

Hi Daniel,

The issue is with the VOCM pin that you have connected to ground. That pin sets the common mode output voltage of the two output pins and is currently trying to drive them to ground. The easiest fix will be to let the VOCM pin float which will set the common mode to half the supply voltage by default. You could also drive the pin with a voltage that is within the pin's range in the datasheet, or you could use split supplies of +/- 2.5V.

Regards,

0 Daniel Berenguer over 7 years ago in reply to Jacob Freet

Expert 2185 points

Thanks so much Jacob.

Indeed, leaving the Vocm pin floating fixes the problem. A last question: what is the difference between leaving the pin floating or connecting the pin to 2.50V in terms of noise rejection or stability?

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THS4551: Simple fully-differential amplifier config