LeCroy DA1855A Operation page 3

Input Resistance
Auto Zero
+ Input Coupling (AC – OFF – DC)
DA1855A-OM-E Rev A
When the input ATTENUATOR is set to 1 and no attenuating
probe is connected, the input resistance can be increased from
1 M to 100 M . This is advantageous when measuring high
impedance circuits or when AC coupling is needed with a very
low frequency cut off. When the input ATTENUATOR is set to 10
or an attenuating probe with read out capability is attached, 1 M
(1M) input resistance is automatically selected.
Unbalanced source impedances can have an adverse effect on
common mode rejection. For example, a differential source with
impedances of 1000 and 2000 , each loaded with 1 M
have a common mode rejection ratio (CMRR) of 1000 to 1. The
common mode rejection ratio can be improved to 100,000 to 1 by
using 100 M input resistance.
Auto Zero is a feature invoked from the user interface in the
Coupling menu when connected via the ProBus interface or in
case the Differential Amplifier is not connected through a ProBus
interface by pushing either the X1 or X10 button, even if a
different gain is not selected. Auto Zero momentarily sets the
input coupling to OFF and determines the offset necessary to set
the output at 0 Volt. During this process the front panel input
signal to the amplifier is interrupted. When the Auto Zero cycle is
completed, the input coupling returns to its previous state. Auto
Zero usually takes less than one second to complete. This
feature allows the user to DC balance the DA1855A simply by
pushing the GAIN button which is already illuminated. When
changing gains, the Auto Zero feature is automatically invoked,
adjusting the amplifier's DC balance.
In OFF mode, the input connector is disconnected from the
amplifier input, and the amplifier input is connected to ground.
The AC coupling capacitor is connected between the +INPUT
and ground through 1 M
RESISTANCE setting. In this mode, the AC coupling capacitor is
quickly charged to the average DC input voltage. OFF mode is
also referred to as precharge mode. Precharge is particularly
useful prior to selecting AC coupling when the input voltage has a
DC component in excess of 19 V. The DA1855A input coupling is
set to OFF and connected to the circuit under test. When the
+INPUT is changed from OFF to AC mode, the coupling
capacitor is already charged, and the trace properly centered on
ISSUED: July 2002
resistor, independent of the INPUT
Operation
will
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