[Design View / Design Solution]
Designing For High Speed In Current-To-Voltage Conversion
As new communications systems reduce the number of RF up-conversions, design of the digital-to-analog stage becomes more challenging.
SFDR to Nyquist offers a glimpse of what to expect for DAC noise. However, only a signal-to-noise plus distortion (SINAD) specification gives the entire story.
Op-amp noise specifications appear as voltage and current noise densities at a particular frequency. Many op amps might seem to give low enough noise levels to be ignored, but it pays to check for the application’s frequency band. In RF applications, the op amp’s 1/f or other low-frequency noise usually isn’t a factor.
Output voltage is deceptively simple. A standard may specify a tight range for the output swing, or you may know the precise level you need to drive something else. Getting the circuit to that level requires a few more answers. What’s the DAC’s full-scale output current for acceptable harmonic distortion? What output load is the op amp driving? Can the op amp drive the right level at the required frequencies and distortion?
Finally, there’s DAC resolution. Quantization error translates into signal-to-distortion ratio for a full-scale sinusoid relatively easily. More resolution will be needed if the output level covers a wide range. The application may have a target SINAD ratio for the low end of the output range plus a maximum level to drive. You’ll need enough resolution for the low end’s SINAD requirement, plus enough additional bits to reproduce the maximum level.
DIFFERENTIAL OR SINGLE-ENDED CONVERSION? The first design choice for a current-output DAC is differential versus single-ended voltage conversion. Preserving differential output with a well-balanced load provides low commonmode distortion and noise rejection. The simplest differential solution is a center-tapped transformer.
In most systems driving a terminated transmission line, a DAC termination resistor will also be necessary. If complex filtering is to be performed on the DAC’s output, driving a transformer directly may not be the best choice.
A dual-supply op amp is a better differential choice when the DAC’s output will be filtered or undergoes further analog processing, or if dc response is needed. In Figure 1, each DAC output drives a 25-O load with 20 mA full-scale. This creates out-of-phase output voltages of 0 to 0.5 V. The op-amp circuit has a gain of one to create a 1-V p-p output.
C1 forms a differential filter with the equivalent 50-O DAC output load. This filter reduces any slew-induced distortion from the op amp, if necessary. This circuit’s high commonmode rejection provides good common-mode noise immunity and cancels some of the even harmonic distortion. Commonmode rejection depends on resistor matching, so 0.1% resistors or better should be used.
The differential op-amp circuit does have some disadvantages. DAC nonlinearity can be affected by voltage-compliance limits at full-scale outputs. Op-amp bandwidth will decrease with gain and higher gain-setting resistor values, meaning more noise.
Op-amp slew rate at the gain used must be fast enough to follow the DAC output. To reproduce a 100BaseTX output signal at full amplitude, the op amp needs to slew at least 300 V/µs. If it can’t, slew distortion will slow waveform edges and generate code-dependent jitter in the output. Edge distortion also results when C1 is used to slow down DAC output so the op amp can follow it.
SINGLE-ENDED SIMPLICITY A single-ended current-to-voltage conversion delivers the best DAC nonlinearity, since the DAC drives a virtual ground. The transimpedance circuit shown in Figure 2 develops a –1-V output across RF from the DAC’s 10-mA full-scale output.
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