Tektronix 11A72 Manual page 13

Affects of Input The AC coupling capacitor of the 11A72 Amplifier isolates both the input termina-
Coupling Mode on tion and the amplifier from extemal DC voltages. You can use the 11A72 Amplifier
DC Circuit Loading with AC coupling selected to safely test circuits that would be damaged by a
50 © load. Avoid inadvertently selecting DC coupling if your circuit will be dam-
aged by the resulting 50 © termination to ground.
The input circuits of the 11A52 and 11A71 Amplifiers are sirniiar to that of the
11A72 Amplifier. In other amplifiers, including the 11A32, 11A33, and 11A34 Am-
plifiers, the AC coupling capacitor does not isolate the input termination from ex-
ternal DC voltages, and will not protect a low-impedance circuit. Figure 1-2 shows
these two types of amplifier input circuits.
~~
©
500
11A72, 11471, 11482
—
iS
11A34, 11A33, 11432
Figure 1-2 — Location of AC Coupling Capacitors In Plug-in Amplifiers
AC Coupling Mode
When AC Coupling is selected, you can test low-impedance circuits that are ele-
vated in potential by up to 100 V DC without loading the circuit at DC. Passive
attenuating probes such as the P6156 or P6057 probes reduce the AC loading
and lower the low-frequency ~3 dB point.
An active probe, such as the P6204 probes, eliminates the possibility of AC
coupling. When an active probe is used, the coupling options are Off and DC.
Off Coupling Mode
When coupling is Off, the AC coupling capacitor charges in about three seconds
to the average DC voltage at the amplifier input. Always use the Off mode when
connecting the amplifier input to a circuit that may have more than 25 V (the
11A72 Amplifiers peak input voltage rating) present. This will precharge the cou-
pling capacitor. Wait a few seconds before selecting AC coupling. The coupling
capacitor is discharged upen disconnection from the external circuit.
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11A72 User Reference Supplement