measurement of differential analog signal with high impedance

[[email protected]]

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

 

[whit3rd]

To measure the high impedance differetial analog input signal, I am
thinking of using 2 FET or BiFET voltage followers

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.

 

[Winfield]

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-.

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.

 

[[email protected]]

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.

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.

 

[whit3rd]

... just DC signal measurement.
I believe the wires between the source and the instument shouldn't be
very long, probably several meters.

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.