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Products - testQube 5220A - DC + AC Voltage Sources

NOTE: this page discusses a product that is currently in development. Features and specifications listed here are subject to change.

Frequently Asked Question: "What is DC + AC Voltage Source?"

Discussion follows:

A DC + AC Voltage Source is the thing you've always wanted but you didn't know you needed! (This might be true only if you've tested analog-to-digital converters [ADCs] and digital-to-analog converters [DACs] in the past...)

For fully testing most ADCs and DACs, you need very stable, very low-noise voltage sources. The vast majority of laboratory power supplies are not adequate for this task. The testQube provides voltage sources whose "DC" drift is less than 0.1 ppm/°C typical and whose noise is extremely low. Such voltage sources are suitable for the reference input of ADCs and DACs. They are also suitable for driving the analog input of an ADC or providing an offset voltage for an external sine generator.

For ADC testing, it would be ideal if an "AC" waveform, such as a sine wave, could be added to the very stable DC voltage. That would allow for generating waveforms to perform "AC" tests on the ADC such as signal-to-noise ratio (SNR), signal-to-noise-and-distortion (SINAD), total harmonic distortion (THD), etc. The DC + AC sources of the testQube 5220A allow an AC waveform to be added to the source's DC voltage setting and the AC waveform, if it's a sine wave, can have frequencies in the 0 Hz to 20 kHz range. This is suitable for testing many industrial ADCs and a good starting point for testing even higher frequency ADCs.

For both ADCs and DACs, the DC + AC source can be used as a reference. In many cases, the reference pin will have a large capacitance to ground (up to many tens of microfarads) and may also have a smaller capacitor that provides a smaller capacitance to ground (such as 100 nF). In these situations, the DC + AC source will only provide a DC voltage. However, it's fairly easy to design circuitry that can disconnect the larger capacitor and keep just the smaller capacitor on the reference pin. This will typically increase the noise of the converter a small amount. The important point here is that an AC waveform can now be applied to the reference pin. If the device is a DAC, the output voltage can be analyzed to see how much of the AC waveform on the reference is getting through to the analog output. If the device is an ADC, "AC" testing with a stable DC input on the converter's analog input pin can reveal how much of the reference's AC waveform is getting into the ADC's digital result. In both cases, the AC waveform on the reference pin can be swept over frequency and the data can provide the rejection of the device's reference over frequency.

The biggest advantage of the DC + AC sources is in the testing of industrial delta-sigma converters with resolutions of 20-bits or higher. The DC + AC sources are effectively required for this type testing and make such things as dynamic testing, normal mode rejection, common-mode rejection, and power supply rejection fairly simple to test (the DC + AC sources can be used as power supplies for very low current devices). For multi-channel delta-sigma devices, one DC + AC source can be used for every analog input channel. This provides an extremely flexible test setup for configuring the device in a variety of ways and can be used for channel-to-channel crosstalk testing, ADC residue crosstalk testing, and many other types of testing. For certain types of delta-sigma converters, particularly those used in power meter ICs, the DC + AC sources can be combined to offer an AC waveform with very precise amplitude control that is summed with a very low drift DC offset. Thus, two DC + AC sources along with some additional analog cicuitry can easily provide single-ended analog signals or differential analog signals with extremely precise amplitude control.

The lowest resolution of the DC + AC sources is 250 nV and this resolution is possible even at the largest output range of ±10 V.


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