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Have I differentially driven the ADA4870 correctly?

Question asked by elecwill on May 15, 2017
Latest reply on May 17, 2017 by JinoL

Hi there,

In a recent project I used the ADA4870 to boost a 2Vpk-pk differential signal to 36Vpk-pk, single ended at the output. 

The input is the analog side of a current steering DAC, connected across two resistors to form a 2V pk-pk differential signal, as per the DAC's datasheet. I connected it to the ADA4870 as shown on the top half of the spice schematic, with Av=18. I realise SPICE simulations are inaccurate in many ways, but I do this with all my designs as a basic sanity check. The simulation amplifies the signal perfectly to 36V, as well as in the single ended case below.

 

DAC to ADA4870 configurations

 

However, when I connected the PCB for testing, the gain was only 1.6 V/V, with and without a load. The amplifier performs exceptionally well in every other regard. The rationale behind the differential input was to take advantage of the 2Vpk-pk the DAC provides vs the 1Vpk-pk when terminated as a single ended output.

 

In order to narrow down potential problems;

Was I right to drive it deferentially or should I have used a single ended input? 

How does the amplifier perform for gains above Av = 10? (The datasheet only goes this high in the examples)

I can convert to a single ended input  by disconnecting R6 and then creatively using some air wires.

 

Many Thanks,

Will

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