SRS Labs SR555 Operation Manual - page 8
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4. Using the SR555/SR556 with a LockIn
Power is supplied to the SR555/SR556 via the 9 pin cable which is supplied with the unit. If it is used as a
pre‐amplifier for a SRS’ lock‐in amplifier, attach one end of the cable to the connector on the rear of the
SR555/SR556. With the lock‐in power off, connect the other end of the cable to the “PREAMP”
connector on the rear of the lock‐in. Both ends of the cable are equivalent. If a longer cable is required,
any standard cable with male 9‐pin D‐sub connectors on both sides, wired pin‐to‐pin, will suffice.
The Output A of the SR555/SR556 should be connected to the (A) Input of the lock‐in. The Output B
should be connected to the (B) Input of the lock‐in. Equal‐length, BNC‐terminated coax cables should be
used for both connections, and for best results the two cables may be twisted together to minimize
inductive pickup. The lock‐in Input Selector should be set to (A–B). Note that the output of the
SR555/SR556 is a balanced, differential signal presented across the A and B outputs, making it important
to receive the output signal differentially.
For applications in which the input current node should be left unbiased (0 V potential), the “Bias
Enable” switch should be left in the down position on the SR555/SR556.
For applications in which a non‐zero potential is to be established at the input current node, the “BIAS
IN” BNC connector is used. This is a DC input that programs the non‐inverting node of the input opamp,
and sets the voltage of input current signal. The DC bias voltage is subtracted by the differential
amplifier and will not be sensed by the lock‐in amplifier.
The overall sensitivity of the lock‐in plus the preamplifier is not displayed on the lock‐in front panel. It is
necessary to divide the displayed sensitivity by the preamplifier gain: 10 V/A for the SR555, or 10 V/A
for the SR556.
5. Using the SR555/SR556 without a LockIn
The SR555/SR556 can be powered with an external power supply. Power is applied through the 9 pin
connector as described in the table below:
Pin Voltage
Current
1
+20 VDC ±10% 80 mA
6
–20 VDC ±10% 50 mA
7,8 Ground
Both DC voltages are required. Either Pin 7 or pin 8, or both may be used as ground. All other pins are
not used. There are two protection diodes connected between the internal regulators and the +20V and
‐20V input pins.