2.2
Instrument overview
2 – 7
resistor of 100
Ω
(10
6
range) or 1 k
Ω
(10
8
range). The maximum full-
scale input signal is 500 nA (10
6
range) or 5 nA (10
8
range). Selecting
AC
input coupling will block the DC
output
of the current amplifier
before it is further amplified by the voltage preamplifier.
Current (transimpedance) amplifiers can be susceptible to noise
peaking or oscillation when driven with excessive input capacitance.
Cable capacitance in particular should be minimized when using the
current amplifier by selecting the shortest cables practical. The SR124
current inputs will remain stable for total input capacitances below
12 nF. Note, however, that external input capacitance will increase
the voltage noise gain of the current amplifier; input capacitance
should always be minimized for best performance.
The overall sensitivity of the SR124 in current mode is dependent on
the Sensitivity setting. The current preamplifier itself converts the
input current signal to a low-level voltage with the specified tran-
simpedance gain (10
6
or 10
8
); the resulting voltage signal is then
amplified by the entire signal chain including the voltage preampli-
fier. The overall full-scale sensitivity is determined by dividing the
Sensitivity setting by the current gain. For example, if the Sensitivity
is set to 20 mV and the input is configured as 10
8
V
/
A, the full-scale
sensitivity will be 200 fA (20 mV
/
10
8
V
/
A).
In most cases, there is little noise improvement for current inputs
by selecting dynamic reserve of
Low Noise
. The greatest stability is
achieved with
High Res
.
2.2.2.3
Grounding
To minimize noise pick-up, it is important to ground the outer shield
of the input cable(s). Grounding the input cable at
both
ends, how-
ever, can in some situations introduce unwanted ground loops to the
experiment. This has the potential to allow stray magnetic flux to in-
duce ground currents to flow through the shield, creating additional
noise and potentially upsetting sensitive measurements.
To help users better manage grounding, the SR124 provides control
of the local grounding of the input BNC shields. When set to
Ground
,
the shields on the A and B input connectors are electrically tied to
the SR124 ground. When set to
Float
, however, a 10 k
Ω
resistor is
added in series between the connector shells and instrument ground.
This 10 k
Ω
resistor is large enough to block flux-generated ground
currents, while still preventing stray charge from accumulating on
the connector shell. If the user’s signal source already provides
a good, low-impedance connection between the signal shield and
ground, then selecting
Float
may eliminate potential ground-loop
problems.
SR124
Analog Lock-In Amplifier
