DAC Reference Feedthrough

# DAC Reference Feedthrough: The Residual Signal at Low Gain

Reference feedthrough is unwanted coupling from a DAC’s reference input to its output. In a multiplying DAC, an AC reference intentionally reaches the output through a code-controlled gain path. Feedthrough is the additional reference-related contribution that can remain when that wanted gain is very small or ideally zero.

Manufacturers may call the relevant specification multiplying feedthrough error. Read its definition, code setting, reference frequency and output-circuit conditions before comparing devices.

## 1. Separate the wanted path from unwanted coupling

Wanted multiplying DAC path and unintended reference couplingConceptual reference input drives MDAC and I-V wanted path to output. A dashed bypass with a capacitor symbol represents unwanted parasitic reference-to-output coupling. At zero wanted gain an AC residual can remain. This is not a complete circuit or a claim of one dominant capacitor.A zero coefficient need not mean zero AC outputConceptual multiplying-DAC path · fixed code, varying referenceREFERENCEAC inputMDAC + I/VWanted gainOUTPUTResidual AC?Parasitic coupling: illustrative feedthrough pathThe dashed route is conceptual, not a complete parasitic model.At an ideal zero-gain code, the wanted path vanishes.Unwanted reference coupling can still leave an output tone.Its amplitude and phase depend on frequency and the circuit.

*Conceptual signal paths. The capacitor symbol represents possible parasitic AC coupling, not an extracted circuit model. An actual result can include several device, package, amplifier and PCB paths.*

For a common inverting multiplying DAC configuration, the ideal transfer is VOUT(t) = −VREF(t)D/2^N. At D = 0, that ideal signal contribution is zero. Real reference-to-output coupling can still create a tone at the reference frequency.

Parasitic capacitance is one mechanism. For a capacitor, current is proportional to the rate of voltage change, so its effect is frequency-dependent. The observed output also depends on the impedances and closed-loop response around that path. A single capacitance and a simple proportional-to-frequency rule do not automatically describe the complete circuit across its full band.

## 2. Quantify the residual before quoting attenuation

Calculated reference feedthrough relative to wanted low code outputAssumed reference-to-output coupling minus eighty decibels with two-volt peak reference gives zero point two millivolt peak residual. Twelve-bit ideal output at code2048is one volt, code128is62.5millivolts, code1is0.48828125millivolts. Residual amplitude ratios0.020percent,0.32percent,40.96percent. At code zero ratio to wanted output is undefined. Assumed code-independent residual for illustration only.A fixed residual becomes significant at low codesIllustrative 12-bit MDAC · 2-V peak reference · residual −80 dBCode DIdeal output peakResidual / ideal20481.000 V0.020%12862.5 mV0.32%10.488 mV40.96%00 VUndefined−80 dB means 0.2-mV peak residual here.Assumed constant residual at one frequency; not device data.Desired and coupled tones combine with their relative phase.

*Calculated example with an assumed code-independent residual at one frequency. Real feedthrough can vary with code, frequency, amplitude and implementation. No named device performance is implied.*

A voltage transfer of −80 dB corresponds to an amplitude ratio of 10^(−80/20) = 0.0001. With a 2-V-peak reference, that gives a 0.2-mV-peak residual. This comparison uses the reference amplitude as its denominator; it is not a ratio to the wanted low-code output.

For an ideal 12-bit MDAC, code 2048 produces a 1-V-peak wanted output from that reference. The assumed residual is only 0.020% of it. At code 1, the wanted output is 2/4096 V, about 0.488 mV peak, and the same residual is 40.96% of that amplitude. At ideal zero output, a residual-to-wanted amplitude ratio is undefined rather than a useful finite percentage.

These ratios compare magnitudes. The wanted tone and feedthrough tone at the same frequency add as complex quantities with relative phase. Do not simply add their peak amplitudes unless assuming the appropriate phase alignment. Cancellation at one frequency or condition is not proof that the unwanted path is absent.

## 3. Distinguish related specifications

Multiplying bandwidth characterizes the intended reference-to-output AC response at a stated code and circuit configuration. It does not establish zero-code isolation. A path can pass a wanted high-code signal well while providing insufficient attenuation near zero gain.

Digital feedthrough is coupling from digital interface activity, commonly characterized without a DAC-core update. DAC glitch impulse concerns an update transient. Reference feedthrough can be present with the code held fixed and digital activity quiet, so those tests do not replace one another.

DC offset and leakage can also create a nonzero output, but they are not the same as a reference-frequency residual. Measure the DC component and the coherent AC component separately. Some bipolar configurations have their ideal zero-gain point at a midscale code rather than code zero; use the correct transfer mapping.

## 4. Measure the intended isolation condition

Apply a clean reference tone at the specified amplitude and frequency. Select the code that gives the intended ideal zero or low gain, allow settling, and minimize unrelated interface activity where possible. Record the output amplifier, feedback network, load, supplies and reference source impedance.

Measure the residual amplitude with sufficient sensitivity and bandwidth. Check the generator’s purity, instrument input limits and measurement floor. Keep peak, peak-to-peak and RMS conventions consistent when calculating the transfer ratio.

Repeat across the required frequency range and critical codes. Inspect phase where cancellation or calibration is being considered. A single zero-code number at one frequency does not establish attenuation accuracy for arbitrary signals. At very small outputs, noise and offset may also matter as much as coherent feedthrough.

## 5. Inspect the entire coupling path

Reference routing can couple directly to output routing or sensitive amplifier nodes. Device and package coupling can remain even with careful board layout. Use the recommended reference-driver and I/V configuration, short appropriate feedback paths and the specified grounding strategy.

Compensation, shielding and layout changes must be evaluated with bandwidth and stability requirements. Adding capacitance to suppress one visible component can change the wanted response or settling. A zero-code measurement can include PCB bypass coupling that is not captured by a converter-only behavioral model.

If wide attenuation range is required, confirm the residual floor in the assembled circuit. Additional muting or switching can improve isolation in some systems, but introduces its own leakage, charge injection, timing and distortion tradeoffs. Verify those effects rather than assuming a nominal mute state gives infinite rejection.

## 6. Set a system-level error limit

Specify the maximum acceptable residual in volts and over frequency, then relate it to the smallest wanted output. Include reference amplitude, code range and measurement bandwidth. For a programmable attenuator, this makes the usable attenuation range explicit.

A useful model keeps intended gain, coherent feedthrough, DC error and random noise separate. That allows a measured limit to guide whether the next improvement belongs in the DAC, output amplifier, reference driver, layout or the overall attenuation architecture.

## References

  • [Analog Devices AN-1085: Multiplying DACs—AC/Arbitrary Reference Applications](https://www.analog.com/en/resources/app-notes/an-1085.html) — multiplying feedthrough and AC application considerations.
  • [Analog Devices AD5415 data sheet](https://www.analog.com/media/en/technical-documentation/data-sheets/AD5415.pdf) — multiplying feedthrough definition and device-specific measurement conditions.
  • [Analog Devices AD5600 data sheet](https://www.analog.com/media/en/technical-documentation/data-sheets/ad5600.pdf) — reference feedthrough and related converter terminology.

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