Help Testing OPA857EVM

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1

------Question for OPA857EVM Please Put your question below------

I am trying to test the OPA857EVM. I am powering the module with 3.3V and I have confirmed that it is supplying > 25 mA using a DMM. I have connected JP1 (test enable) and JP2 (gain control) both to Vcc. I am driving J1 (test in) with a 50 mV amplitude, 2.1 V offset, 1000 kHz signal, which shares a ground with the power source. I am reading from J3 (Vout diff) to an oscilloscope with a 50 Ohm terminator. I am expecting a 500 mV amplitude sine wave output. However, I don’t see anything on the oscilloscope.

Thanks!

2

Hello aileenzhang and welcome to the DigiKey TechForum

I am reaching out for some possible solutions. We will let you know when there is an update.

3

This looks a bit different from the typical op amp, and I’m thinking that the documentation might have some room for improvement.

The datasheet’s functional block diagram suggests that the OUTN is basically a buffered, supply-derived reference voltage and that the actual amplifier output has a rather narrow output swing

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I don’t really see any indication here that the DC bias of the test input signal isn’t getting the same 10x gain resulting from the 2K input impedance model working together with the 20K configured transimpedance gain. Tweaking the DC bias to put the OUT pin in the middle of the listed range before adding the AC component might be a thing to try.

Looking at the EVM schematic it seems that R5-7 form roughly a 10x voltage divider, and if the output is loaded with 50Ω one could probably expect something closer to an overall gain of 1 from the TEST IN pin to the Vout at J3.

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4

Thanks for the reply! I was planning to try out modifying the DC bias. However, when I was testing, I noticed that when I connected the test signal to J1, the signal was destroyed. When it was disconnected, I could see the sinusoidal signal that I wanted to drive with. However, as soon as I connected the signal to the board, it turned into 0.7 V of DC signal. Is this expected? I tried modifying the DC bias and the amplitude of the sine wave, but there was little to no effect on the input signal.

5

R4 would certainly load a high-impedance source, but I’m wondering if there may be an issue of unintended connection occurring. Take a careful inventory of how all your test equipment is connected, noting the potential for unexpected connections through the AC utility ground.

6

Have you solved this problem? I have the same problem as you. I tried the test mode but there is no signal from the output.