Tektronix ADA400A Instruction Manual page 20

Reference
10
When you must use any length of unshielded wire, twist the wires together and
connect the wires differentially. Twisting the wires together helps to introduce
any electrical interference as common-mode noise that the differential preampli-
fier cancels out with a differential setup on the inputs.
For other tips on reducing signal interference, refer to page 17.
Because the ADA4O00A Differential Preamplifier is designed to amplify small
DC and AC voltages, do not connect voltages that exceed the input rating.
CAUTION. To avoid damaging the inputs of the ADA400A Differential Preampli-
fier do not apply more than 40 volts (DC + peak AC) between any one input and
ground or 80 valts (DC + peak AC) differential between the inputs.
To prevent damage to the input circuitry from the misapplication of large
voltages, a fuse on each input opens if the input current exceeds 62 mA (1/16 A).
To replace or check these fuses, refer to the procedure on page 47.
Each input connector of the ADA4O0A Differential Preamplifier has a character-
istic input impedance of 1 MQ to ground in parallel with approximately 55 pF.
For low source impedances, this | MQ is more than adequate to prevent the
inputs from loading the signal source.
If impedance of the signal source approaches 1 MQ, the impedance of the inputs
begin to load the signal source and reduce the amplitude. In effect, the extremely
low current of the signal source is not able to develop a voltage across the 1 MQ
resistance and there is no signal to amplify. In this case you may want to select
the infinite Q position.
Infinite QO. To measure signals from high impedance sources, you can internally
disconnect the 1 MQ resistance of the ADA400A Differential Preamplifier. and
select the infinite Q position. This position prevents the preamplifier from
loading a high impedance signal source.
Observe the following operating limitations when you select the infinite Q
position:
m= Select either the X100 or the X10 gain position. Operation in the 0.1X and
1X gain results in gain errors.
= Ensure that the source has some impedance to ground. The internal input
amplifier requires some input bias current to operate. Typically this amount
of input bias current is less than 25 pA.
ADA4OOA Instruction Manual