Stanford Research Systems SR865A, Operation Manual

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66 Operation Chapter 3
SR865A DSP Lock-in Amplifier
coupling results in gain and phase errors at low frequencies as wells as reduced CMRR
performance.
Remember, the Reference Input is ac coupled when a sine reference is used. This also
results in phase errors at low frequencies.
Voltage [Ground]
This key chooses the shield grounding configuration. The shields of the input connectors
(A and B) are not connected directly to the lock-in chassis ground. In Float mode, the
shields are connected by 10
kΩ to the chassis ground. In Ground mode, the shields are
connected by 10
Ω to ground. Typically, the shields should be grounded if the signal
source is floating and floating if the signal source is grounded. Do not exceed 1 V on the
shields.
Voltage [Input Range]
The voltage input range specifies the largest input signal (ac or dc coupled) before the
voltage input is overloaded. The smaller the input range value, the higher the amplifier
gain leading to the A/D converter. The signal strength LEDs indicate how much of the
A/D range is being used. When the lowest yellow LED is on, try decreasing the input
range (increasing the gain).
In general, use the smallest input range possible without overload. Remember, the largest
signal, whether it’s at f
ref
or just noise, will be the first to overload.
This setting has no effect when the current input is selected.
The Signal Monitor on the rear panel is the amplifier output.
Current [Range]
The current input uses the I connector. The input burden resistance is 1 kΩ (10 nA range)
or 100 Ω (1 μA range ) to a virtual ground. The shield is chassis ground. The largest
allowable current (ac plus dc) before overload is approximately ±4 µA or ±40 nA. No
current larger than 10 mA should ever be applied to this input.
The current range determines the input current noise as well as the input bandwidth. The
10 nA range has 10 times lower noise but 200 times lower bandwidth. Be sure the signal
frequency is below the input bandwidth limit. The noise and bandwidth are listed below.
Range Noise Bandwidth
1 μA 130 fA/√Hz
400 kHz
10 nA 13 fA/√Hz 2 kHz
The impedance of the current source should be gre ater than 1 MΩ when using the 1 μA
range and greater than 100 MΩ when usi ng the 10 nA range.
The signal strength LEDs indicate how much of the A/D range is being used. The Signal
Monitor on the rear panel is the amplifier output.
The current range setting has no effect when the voltage input is selected.