# What is the input impedance/resistance of TDA7293

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#### garybdmd

Hi all,
The datasheet for the TDA7293 says its input resistance is minimum 100k, typical and maximum are not listed. Here's a link: http://www.st.com/web/en/resource/technical/document/datasheet/CD00001887.pdf It looks like the positive, non-inverting input goes right into an op-amp on the chip. Should the typical input impedance be several megaohms or so? I was needing to make an amp with a high input impedance about 1 meg ohms and was hoping this would be a good choice.
Thanks,
Gary

#### JonSnell Electronic

The nominal input would be 22k, so use a Fet input op amp as impedance matcher. TL071/2/4 is a good choice, with a gain of 1.

#### AndrewT

R1 sets the input resistance. It usually matches R3.

#### milliganp

To understand the input of the amp look at the input bias current; this is 0.3uA typical 1uA maximum on the datasheet. This indicates a bipolar (as opposed to FET) input. The input bias current runs through the input ground resistor, which on the datasheet 'typical' circuit is 22k. This will produce an input offset of -6mV to -22mV (the bias current has to flow through the 22k resistor. If you tried to increase the impedance to 1M the offset would become -0.3V-1V so increasing the value is NOT a good idea.
As per other suggestion, if you need high impedance add a buffer based on a FET op-amp.

#### AndrewT

if one places the SAME resistance value on the -IN and +IN pins, then the two input offsets due to input bias current cancel.
The result is zero output offset.
If one replaces one of the input bias resistors with a variable resistor then one ends up with a variable output offset.

It's errors in chip manufacture and bias resistors that give output offsets. These can be minimised by good component selection and using a good topology.

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#### garybdmd

thanks guys all for the insight!

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