Is DC offset present with AC Interface when connect to an external power amplifier?
AT3600-AC Interface Operation:
In normal operation the AC Interface and AT3600 are in a closed loop (in the amplifier/analog control setup). The 0-5V output voltage from the ESD signal is amplified by a power amplifier and placed across the DUT. An algorithm within the AT3600 automatically trims the ESD control signal to achieve exactly the correct applied voltage in the test program. This 0-5V ESD voltage level for a specific test voltage will therefore vary dependent on the specific amplifier gain and test conditions.
If the external power amplifier is disconnected (with the amplifier/analog control setup) the measured ESD voltage is in an open loop state and not in a well defined state. Under certain conditions this may result in a higher than normal DC level.
An example of this is shown below with a VOCX test with primary voltage set to 100Vac.
Voltech test equipment setup:
Test setup: AT3600, rev 3.20 firmware, s/n
AC Interface module s/n 9559
Krohn-Hite Power Amplifier Model 7500 (gain 100)
The test program and external power amplifier setup are shown the an attachment entitles “AC Interface VOCX test”
1. Closed loop measurements with Krohn-Hite connected (AC coupling)
Test results with Krohn-Hite AC coupling and AC voltage in test program:
Vin(set) = 100Vac, Vac(measured) = 99.9Vac
2. Closed loop measurements with Krohn-Hite connected (DC coupling)
Test results with Krohn-Hite DC coupling and DC voltage in test program:
Vin(set) = 100Vac, V(measured) = 1.35Vdc (0.0135Vdc at ESD output)
(Note power amplifiers are normally AC coupled and this will not apply)
3. Open loop measurements with Krohn-Hite disconnected:
ESD output = 0.21V (test program running but hanging due to no power amplifier)
The DC voltage at the ESD output is 0.0135Vdc with the power amplifier connected, this is the normal test condition. However with the power amplifier disconnected the DC voltage at the ESD output is 0.21V, however this is not a normal condition however.
Please note: power amplifiers used with Voltech testers are transformer AC coupled and this DC content is not normally seen at the transformer.
Therefore, the system results in closed loop (normal operation) are correct and as expected.
If the power amplifier introduces significant DC content it may be advisable to add an isolation transformer at the output to remove DC content.