Measuring a Multiplying DAC with an AC Reference Signal

A multiplying DAC can use its reference input as an analog signal path while the digital code controls amplitude. That application places different demands on the circuit from a fixed-reference DC output. Evaluate the reference path, output amplifier, and residual feedthrough across the intended frequency and code range.

Define the multiplication mode

The Analog Devices application note AN-1085 distinguishes multiplying bandwidth, analog distortion, and reference-to-output feedthrough. It explains that the output amplifier and compensation contribute to the AC performance of the complete multiplying circuit.

Record reference amplitude and offset, digital code range, output configuration, amplifier supplies, and load. Confirm that the reference signal remains within the device’s permitted input conditions.

Measure gain versus frequency

Hold the digital code fixed and sweep reference frequency. Measure the actual signal at the DAC reference pin as well as the output so source loading is included correctly.

Repeat at several codes, including low-amplitude settings. Normalize each result to its intended gain rather than comparing only absolute output amplitude. A circuit can have acceptable full-scale response while its useful attenuation range collapses at higher frequency.

Inspect zero-code feedthrough

Load zero code and retain the AC reference input. Measure the residual output with sufficient instrument dynamic range. Establish the measurement floor by checking the setup’s own coupling and leakage.

For example, moving the reference cable farther from the output cable may change a small measured residual. That observation helps distinguish fixture coupling from feedthrough inside the DAC circuit.

Check distortion and large-signal limits

Measure harmonic content at the largest intended reference amplitude and relevant frequencies. Compare smaller amplitudes to determine whether a limit is associated with signal swing, slew behavior, or another mechanism.

Check the output amplifier’s operating point and loading. A suitable bandwidth headline does not guarantee adequate large-signal performance in the actual current-to-voltage configuration.

If the digital code changes during operation, evaluate those transitions separately from the fixed-code AC sweep. Code-update glitches and reference feedthrough are different effects and can require different remedies.

Repeat key measurements at supply and temperature conditions that affect the required performance. Preserve compensation values and the exact amplifier choice in the record.

Save gain-versus-frequency curves, distortion, zero-code residual, and measurement-floor checks. Define the useful attenuation and frequency range from these results. A DAC’s resolution describes its code granularity; it does not guarantee that every low code produces the intended small AC output when feedthrough and amplifier limitations become significant.