Created on 2009-03-06 11:16:00
I need to test my device with a combined AC/DC signal. Can I generate this signal with my 2700 Series audio analyzer?
The System Two Family instruments (AP2700 (2722-192K), Cascade Plus (2722), Cascade (2522), and System Two (2322)) can generate Analog output signals that can be configured for Balanced Floating, Balanced Grounded, Unbalanced Floating, or Unbalanced Grounded. To meet the design requirements for the floating and grounded analog output configurations, a signal transformer was designed into the analog generator output stages. A transformer does not allow DC to pass from the primary side to the secondary side.
Applying external DC voltage to the instrument’s analog generator outputs will activate output protection circuits and damage other internal components, and is therefore not recommended.
DC voltages can safely be applied by adding an additional transformer externally to isolate the analyzer’s outputs from the applied DC voltages. The external transformer’s primary is driven by the analyzer’s analog generator in the balanced output configuration. The secondary can be configured for balanced or unbalanced output. An external DC power supply provides a voltage offset that can range from a few millivolts to as high as 100 volts. Make sure that the DC voltage applied is well under the breakdown voltage rating of the transformer. The power supply should have current limiting to protect the transformer and DUT in the case of a fault.
Audio Precision recommends the Jensen JT-123-BLCF transformer (www.jensen-transformers.com) for most applications where low THD signal generation and measurement is desired. The transformer has split primaries and secondaries, which allows you to create a center-tap for injecting DC voltage in the balanced output configuration. When feeding a high impedance load, the DC current running through the secondary will generally be too small to magnetize the core and cause any problems.
Even though the recommended transformer is very high quality, any externally added components in the signal path are going to have some effect on the output signal. We recommend that you measure baseline performance before performing any tests by feeding the external circuit output back into the analyzer input. Make sure that signal level, DC voltage, and load impedance mirrors the actual test conditions. Of note is that transformer distortion tends to increase at low frequencies, so distortion measurements should be swept or measured at multiple points.
A high quality film capacitor is recommended for C1, with the value depending on the input impedance of the DUT. With an input impedance of 10 KOhms, a 4.7μF capacitor will result in a 3dB roll-off point of 3.4 Hz. However, since there will already be smoothing capacitors on the output of the power supply, the roll-off point may actually be much lower.
Note that the method described in this article is suitable for injecting an audio signal with a DC bias, but is not suitable for sourcing significant current due to the resultant transformer saturation. For applications that require more current, there are industrial power amplifiers that can pass both AC and DC, DC power supplies that can accept an external AC input, and all-in-one power supplies that have a built-in sine wave generator. The all-in-one solution of course can not generate the variety of signals that the System Two Family instruments can.
AC plus DC balanced output configuration