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measurement of differential analog signal with high impedance

Hello!

I asked something similar before but I think it is best to open another topic as it is bit different.

To measure the high impedance differetial analog input signal, I am thinking of using 2 FET or BiFET voltage followers for each input (V+ and V-), and then connecting voltage followers outputs to ADC's Vin+ and Vin-.

Is the above circuit right? Any recommended parts for the voltage follower (or so called ADC buffer) and the ADC ? The analog signal is between +/-5V and the ADC has to be a 16-bit resolution one (requirement); It is best to have single power supply for buffer and ADC; and I need at least 16 differential channels to measure.

Can someone help?

Thanks

Reply to
DAXU
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Sounds good, but it seems you're talking about IC followers (integrated amplifier chips); discretes are better for preamp applications, usually.

'High impedance' means that there is some acceptable approximation that one impedance is much higher than another. Is your source to be the 'high impedance'? Or the load? What bandwidth do you require? Is DC important? Are there long wires involved between the measuring instrument and the source?

Highest impedance amplifiers are vacuum tubes. Noise performance is poor if you ask for TOO high an impedance, or insist on large bandwidth.

Reply to
whit3rd

It's good to consider taking the input difference and removing the common-mode voltage before presenting the signal to the ADC, even if it is a differential ADC. Look up instrumentation amplifier ICs. Many of these contain low-offset JFET input follower opamps plus a high-performance output difference amplifier. And they let you amplify the output voltage if you wish. Analog Devices and Burr-Brown (now TI) are two good sources for instrumentation amplifiers.

Reply to
Winfield

Thanks for your reply. The source is high impedance to the ADC input. No bandwidth requirement, just DC signal measurement. I believe the wires between the source and the instument shouldn't be very long, probably several meters.

Reply to
DAXU

The least expensive option would be multiplexing the inputs to a single amplifier; this will add some high-frequency requirement to the situation, due to the multiplexer switching, and multiplexer feedthrough/leakage currents can be troublesome. Better but pricier would be to have an instrumentation amplifier on each channel, and multiplex the amplifier output; you say there are lots of inputs, so IC instrument amps are the easy option. They're mass-produced and pre-trimmed. Something like INA2126 might be ideal.

The 'few meters' of wire is a potential problem. Shielded twisted- pair wires are excellent. Unshielded twisted pair is next best. Some input filtering will be important, too, because even radio pickup can affect the low bits of a 16-bit converter.

Reply to
whit3rd

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