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Amplifier Answers
This page contains answers to frequently asked questions about our
amplifier products and about signal amplification in general.
For a list of the questions, please return to the main
FAQ Index
page. If you still have questions after reading this page,
email them to info@symres.com
and we'll be glad to answer them for you.
Q: Why would I need an amplifier with a 24 bit A/D converter ?
An amplifier can serve two useful purposes. First, it can increase
the input sensitivity (counts/volt) of the A/D. Second, it can
increase the input impedance of the A/D.
With the PARxCH series of products, the full differential input
sensitivity is: 40v / 2**24 = 2.38 microvolts per count. And
actually, if you have a noise floor at 21 bits, then it is 40v /
2**21 = 19.07 microvolts per count. For some applications this is not
enough sensitivity. Adding in an amplifier with a gain of 100
improves the 21 bit level sensitivity to 190 nanovolts / count. Of
course the amplifier also reduces the maximum differential input
voltage range to +/-100 millivolts.
In regard to input impedance, the normal impedance of the PARxCH
inputs are 16k ohms. For many applications this is a good match.
But, there are applications where higher input impedances are
desirable. With the AMP4CH-DF included in the signal processing path,
the input impedance can be set at any value desired. The amplifier
inputs are non inverting op amp inputs with impedance in the 100+ Meg
ohm range. There are provisions on the board for additional resistors
in parallel with the inputs to reduce the input impedance to any
suitable level. Normally we populate the parallel resistors with 51k
ohm.
Do not assume that a higher input impedance is better. If you are
running cabling from an active sensor to the amplifier inputs, then
lower input impedance on the AMP4CH-DF or PARxCH is actually better.
With a lower input impedance, the currents from your signals are
larger and compete less with the noise currents from 50/60Hz noise and
leakage. The larger the signal is with respect to the background
noise the better. Of course if you are using a passive sensor, a
higher input impedance may be required so the sensor is not loaded
excessively. In those cases it is usually a good idea to experiment
to find the best match between signal current and sensor loading. The
parallel resistors on the AMP4CH-DF make it easy to optimally tune the
system.
Q: How does an amplifier affect the noise level of acquired data ?
It is a basic fact that as an amplifier's gain increases it will add
more noise. At gains of 100 or less with the inputs shorted the
AMP4CH-DF does not add any noise that we can detect. At a gain of
1000 you will see the amp itself contribute noise.
In fact, beyond a gain of 100 with the AMP4CH-DF, it is almost a zero
sum game. For example, if you increase the AMP4CH-DF gain to 200, you
will see approximately one additional bit of noise on the A/D display
in scope. At first, one might be happy saying they can see smaller
signals at a gain of 200. But, if the amp starts contributing one bit
to the output noise floor, you really are effectively running with a
gain of 100. At a gain of 100 the AMP4CH-DF does not add to the noise
floor, yet still provides the benefits of an amplifier.
Also note that with the inputs shorted you will see one noise floor,
while with the inputs open you will see another. This is mostly
controlled by the 51K resistors in parallel with the inputs on the
AMP4CH-DF amplifier board itself. At these signal levels the Johnson
thermal noise of the input impedance becomes a consideration.
Changing the 51K resistors or the impedance of the driving signal will
correspondingly change the Johnson noise contribution.
Q: What type of analog input cable is best for the AMP4CH-DF ?
At a gain of 100 in front of a 24 bit A/D converter, signal levels are
small enough that flat ribbon cable becomes susceptible to 50/60Hz
noise pickup. In fact, even "twisted ribbon" can have 50/60Hz
problems because it has flat stretches of cable. When 50/60Hz flux
lines cross open loops of cable, noise voltages are generated by
Faraday's law.
Discrete twisted pair cable is ideal for analog inputs to the
AMP4CH-DF amplifier. A simple round shielded twisted pair cable is
available from Belden Wire for about $22/100ft. Actually, even
unshielded twisted pair made with single wires and a drill is very
effective. For best results make sure the twist runs all the way from
your sensor to the AMP4CH-DF inputs with no flat untwisted stretches.
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