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4
1.6 µF. The real portion of the input
impedance (0.5 Ω) determines the noise
performance. See page 7 for detailed
information on input impedance.
The input can be floated up to ±100 VDC
with respect to the chassis. The
maximum AC input before overload is 14
mV RMS (±20 mV peak) when the unit is
in the buffered mode. In either mode, the
input is clamped at about 350 mV RMS
(±0.5 V peak). Care should be taken when
the unit is used in the bypassed mode,
since a ±0.5 V peak input becomes ±50 V
at the output.
In the buffered mode, the OVERLOAD
indicator will light when the preamplifier
overloads. An overload that occur after
the preamplifier will be indicated by the
lock-in amplifier’s overload indicator. The
SR554’s overload indicator is only
functional when the unit is in the buffered
mode. If the unit is operated in the
bypassed mode and power is applied, the
overload LED may light even when the
unit is not overloaded. This does not
indicate an overload, but is due to leakage
current of the buffer amplifier and its
protection circuitry. To avoid this, power
should not be connected when the unit is
used in the bypassed mode.
OUTPUT CONNECTION
For single ended operation the (A) Output
of the SR554 should be connected to the
(A) Input of the lock-in amplifier. The
center conductor carries the signal and
the shield is ground. For most
applications, this single connection will be
adequate.
For situations with potential noise pick-up
on the cable, it may be better to operate in