Signal Channel Inputs - Ametek 7280 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION
Figure 3-1, Model 7280 - Block Diagram

3.3.02 Signal Channel Inputs

The signal input amplifier can be set for either single-ended or differential voltage
mode operation, or single-ended current mode operation. In voltage mode a choice of
AC or DC coupling is available using an FET input device. In current mode a choice
of three conversion gains is available to give optimum matching to the applied signal.
In both modes the input connector shells may be either floated via a 1 kΩ resistor or
grounded to the instrument's chassis ground. These various features are discussed in
the following paragraphs.
Input Connector Selection, A / -B / A - B
When set to the A mode, the lock-in amplifier measures the voltage between the
center and the shell of the A input BNC connector, whereas when set to the A-B
mode it measures the difference in voltage between the center pins of the A and B/I
input BNC connectors.
The latter, differential, mode is often used to eliminate ground loops, although it is
worth noting that at very low signal levels it may be possible to make a substantial
reduction in unwanted offsets by using this mode with a short-circuit terminator on
the B/I connector, rather than by simply using the A input mode.
The specification defined as the Common Mode Rejection Ratio, C.M.R.R., describes
how well the instrument rejects common mode signals applied to the A and B/I
inputs when operating in differential input mode. It is usually given in decibels.
Hence a specification of > 100 dB implies that a common mode signal (i.e. a signal
simultaneously applied to both A and B/I inputs) of 1 V will give rise to less than
10 µV of signal out of the input amplifier.
The input can also be set to the -B mode, in which case the lock-in amplifier
measures the voltage between the center and the shell of the B/I input connector. This
extra mode effectively allows the input to be multiplexed between two different
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