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# TLV2374: Lost gain in non-inverting stage

Part Number: TLV2374

I have designed a USB oscilloscope.

On the input I have a 1 meg resistor going into the non-inverting input of a unity gain non-inverting stage.

I was expecting to get out what was on the input of the 1 meg resistor but it is down about 50%.

The op amp input current is 1pA so I cant see that affecting my circuit.

• Sounds like a possible voltage divider or impedance mismatch. Can you share the relevant portion of the schematic?

Some further questions:
* Is the error always 50%, regardless of input voltage?
* What load is on the output of the unity gain buffer?
* What is the amplitude and frequency of the input signal you're testing?

-Paul
• The circuit is very simple just a 1 meg resistor going to non-inverting input .

The op amp stage is unity gain. See picture.

I put a 5 volt DC signal into the input and there is 2.5 volts lost across the 1 meg resistor.

If that is true then the input current to op amp is 2.5uA when spec says it should be about 1pA typical.

• Hi Nigel,

shouldn't it be this way?

Kai

• No, the circuit is meant to be the front end of an oscilloscope and the 1 meg protects the op amp as the input can be as high as 160 volts.
I am going to add an attenuator circuit after the 1 meg to bring down the input voltage.
• I applied 5 volts to input and power supply is only 3.3 volts so that's why I only got a couple of volts out. Sorry !

I checked again with an AC signal and that is about 50% down at 20KHz.
So its probably more like a bandwidth problem or input capacitance problem.
• I looked up the datasheet and it is 8pf input capacitance hence the 50% down at 20KHz.
All makes sense now.
• 10pf across 1 meg resistor lifts higher frequencies nicely.