Decimate Signed

Controls: X-Axis Dialog >> Slow >> Signed
Macro: X_Sign

As described under the Demodulate topic, that mode suports a simple external chopper scheme that allows a standard AC-coupled sound card to record DC or very slow signals, like temperature or atmospheric pressure. But it is limited to unipolar signals only, since negative inputs produce the same result as positive inputs.

To properly handle signed DC inputs, the Signed option supports a more complex external circuit with four switching states: A positive reference voltage, then an inverted version of that reference, then the raw input voltage, then the inverted input. This 4-state cycle repeats continuously.

The reference level is chosen to be greater than any valid signal, allowing Daqarta to deduce which phases are which, and thus to infer the correct polarity of the input signal. See the complete circuit and operation description in External DC-to-AC Modulator.

To use Signed, you set Decimate on with a Decimate Rate that is less than the overall 4-state cycle rate. The example circuit has an overall rate of 1 Khz (after division down from a 4 kHz clock), so you must set the Decimate rate lower than 1 kHz. The operating frequency is not especially critical, so the circuit uses a simple oscillator that is subject to component tolerances and drift. Thus, it's best to use a safety factor here, such as setting Decimate Rate to 500 Hz or less.

You will probably want to run with Trigger off due to the low effective sample rate. This will give a scrolling waveform like a chart recorder or data logger, so the trace remains "live" without long waits for trigger events.

But if you do use Trigger with Signed, note that the triggering operation is applied to the decimated data, not to the raw data. The Trigger Source label becomes a button marked 'Source Decimate', which (like Demodulate, but unlike Envelope or plain Decimate mode) can't be toggled to 'Source Raw'

See also Slow (Decimate) Controls, X-Axis Control Dialog