SRS Labs SR555 Operation Manual - page 7
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3. Description
The block diagram below shows the SR555/SR556 current amplifier which converts input current into a
differential output voltage. The total differential output voltage is given by
. A current
amplifier is also called a Transimpedance Amplifier, or a TIA.
Figure 7 A block diagram of SR555/SR556 current amplifiers
Unlike a simple resistor, the transimpedance amplifier presents a low‐impedance node to the source.
This node is generally a virtual ground. In the SR555/SR556, this node voltage can be set at the user
provided voltage by switching to “Bias Enable”. This bias voltage will not be present in the output signal
since it is subtracted out by the post differential amplifier.
The SR555/SR556 can be located close to the source to eliminate long cables, capacitive loading and
noise pickup. The two output signals should be used differentially for best results. The capacitance from
long input cables or intrinsic capacitance from the input source will add up to the total source
capacitance . This will react with the opamp voltage noise e
n
(shown in Figure 5) to add an effective
current noise of 2
to the equivalent input noise, as shown in Figures 3 and 4 (so‐called “noise
gain peaking”). It should be noted that has virtually no effect on the signal gain, as shown in Figure 1.
The SR555 and SR556 are ideal low‐noise current preamplifiers for SRS’ lock‐in amplifiers, and can also
be used for any other small current detection purposes.