Differential and Common Mode DC Offset with APx

Created on 2015-06-24 13:00:00

Usually an analog audio signal contains just an AC waveform with no DC component. There are, however, cases where a DC voltage is present, either intentionally (microphone bias) or unintentionally (excessive op-amp offset, failed blocking capacitor or power supply, etc.). The adjustable DC offset feature on APx Series analyzers allows you to generate an AC sine wave with a DC offset, to measure the effects of DC offset on the audio quality of a device under test.

2-channel analog output module showing normally wired balanced cables and AC and DC polarity.

Figure 1: APx 2-channel balanced output normal cabling, for differential mode DC offset.

The AC audio signal on a balanced line is normally in differential mode, where the polarity of the voltage on the inverted phase (pin 3 or “lo” on an XLR connector) is opposite that of pin 2 (“hi”). However, when adding DC offset to a differential AC signal, the DC voltage can similarly be applied in differential mode, or alternatively in common mode (same DC polarity on pins 2 and 3).

Generator panel, configured as follows: waveform sine plus offset, level 100 mV, frequency 1 kHz, and offset plus 5 volts.

Figure 2: Level and Gain measurement generator panel, with + 5 vdc offset added.

The DC offset added to the balanced output signal on APx instruments is differential mode. The polarity of the offset setting in the APx500 software corresponds to the polarity of pin 2.

There are cases though when it is desirable to generate a differential mode AC audio signal, but with a common mode DC component. This is often required for dynamic range measurements of A-to-D converters with differential inputs. This can be accomplished on APx analyzers by using a set of special cables that swap pin 3 on channel 1 with pin 3 on channel 2, and then inverting the DC offset voltage setting on channel 2. See Figure 3.

2-channel analog output module showing special wiring, pin 3 channels swapped

Figure 3: APx 2-channel balanced output special cabling for common mode DC offset with normal mode AC.

Advanced settings dialog with ac level and dc offset settings for each channel

Figure 4: Advanced Settings dialog, + 5 Vdc set on channel 1 and -5 Vdc on channel 2.

Since pin 3 on both channels carries an identical AC signal, the special cable wiring has no effect on the sine wave test signal. However, as pin 3 on channel 2 now has a positive voltage, when combined with the positive voltage on pin 2 of channel 1, we get common mode DC. In this configuration, you’ll have positive common mode DC on channel 1, and negative common mode DC on channel 2.

To setup the DC Offset in APx use the Advanced Settings to set the two channels independently for the AC Level and the DC Offset (Figure 4). The DC common mode voltage output will be 50% of the values entered for DC Offset, which in this case with +5 V offset for Ch 1 and – 5V offset for Ch 2 results in +2.5 V with respect to ground on each pin of the special cable.

If you are using an APx515 or APx525 family analyzer with two output channels and desire to have two channels with the same DC polarity, split one output of the special cable and leave the other output disconnected. With an APx585 or APx586 analyzer that has eight output channels, you can have up to four discrete outputs of each polarity before adding any splitters.

2-channel analog output module showing normally wired balanced cables and AC and DC polarity.

Figure 5: APx 2-channel balanced output normal cabling, CMTST configuration for common mode DC offset.

If you want to test AC common mode rejection ratio (CMRR) while presenting a DC offset, do not use the APx500 CMRR measurement, as the DC voltage on pin 3 will get reversed mid-test. Instead, in signal path setup, set the Analog Balanced Output Settings Configuration control to CMTST (Figure 6) and use the Level and Gain measurement.

2-channel balanced output configuration CMTST setting for AC and DC common mode rejection measurements

Figure 6 APx 2-channel balanced output configuration CMTST setting for AC and DC common mode rejection measurements

The CMTST configuration puts the same DC and in-phase AC signal on pins 2 and 3, so DC offset will be common mode and no special cable is required (Figure 5). With this configuration the actual common mode DC voltage is the full DC offset setting (not 50%).

To do differential mode DC with common mode AC in the CMTST configuration, wire the special cable as before (figure 3 above) and set DC offsets as before.

Special Caveats

1) Both channels in a channel pair configured for common mode DC offset must have equal and opposite polarity DC Offset level settings.

2) The analog generator maximum AC plus DC output voltage is constrained by hardware.

  • This varies by APx model. For example, the APx555 balanced analog generator maximum output AC + DC is ± 18.85 V. A sinewave output with a level of 18.85 Vp cannot be offset with a DC voltage. However a 10.0 Vp sinewave can be offset with an 8.85 V DC offset. The APx software will constrain these settings.
  • For the configuration in Figure 3, the maximum common mode DC output voltage is 50% of the equal and opposite polarity DC Offset settings.

3) High levels of DC offset with respect to the AC level will degrade the sinewave generator distortion.

  • This is true because the DAC output range is constrained by the combined AC peak level and DC offset level.
  • Generator THD+N distortion performance will decrease as AC level goes down and DC offset goes up. This is especially true of the Dynamic Range measurement that uses a -60 dB sinewave by default.
  • Generator distortion performance is not guaranteed with this method.

4) The APx555 DAC generator may be used with this method, however the High Performance Sinewave Generator does not support it (no DC Offset capability).