Tektronix ADA400A Instruction Manual page 24

Reference
14
Precharging the Signal
Gain, Input Range, and
GND Coupling. The GND position decouples the signal from the input and
couples the internal input of the preamplifier to ground. This position also
precharges the AC coupling capacitor to the DC level of the signal. (See the
DC Coupling. The DC position couples all signal components, AC and DC, to the
preamplifier. (You can adjust the offset controls to null out DC biases.)
Follow the procedure below when coupling AC signals that have a high DC
voltage level or when probing between signals that differ greatly in DC levels.
This procedure becomes especially useful when you disable the | MQ input
voltage.
1. Set the input coupling switch to GND before connecting the input to a signal
2. Connect the preamplifier input to the signal source.
3. Wait two seconds for the input coupling capacitor to charge to the DC level
of the signal source.
4. Set the input coupling switch to AC. Position the AC signal within the
The gain selector sets amplification factor of the preamplifier to X0.1, X1, X10,
a common-mode (dynamic) range, and variable offset range. To prevent the
signal from distorting or "clipping" (see Figure 10) you must use the appropriate
gain setting and offset.
The amount of differential, common-mode, and offset range depends on the gain
setting. Figures 8 through 11 beginning on page 15 show the common-mode,
The OVERRANGE indicator lights when the input signal is outside the
differential input range for a particular gain setting. This light indicates that the
output signal is distorted or "clipped" and the oscilloscope display may not be
accurate. Try a lower gain setting or check the position of the offset controls if
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