[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.
For a 1-V output, choose 20 mA full-scale from the DAC. Configure the op amp for unity gain with the values in Figure 1. Not all high-frequency op amps are unity-gain-stable, which is important for highest bandwidth in a differential application. The ADA4899-1’s unity-gain –3-dB bandwidth is about 120 MHz for a 1-V output.
Using the lower half of Figure 3’s circuit with C1 = 0 gives a pi-section filter with a low-pass voltage transfer function. The approximate –3-dB rolloff point is 123 MHz, so the op amp sets the response limit in this example. Op-amp settling time may set a lower limit to maintain required harmonic levels.
The op amp and its resistors contribute noise to the output in both circuits. The op amp’s input-referred voltage noise density VN is 1 nV/vHz. Its input-referred current noise density IN is 2.6 pA/vHz. Each circuit has three noise contributors: voltage noise from the op amp, voltage noise from op-amp noise current through the resistors, and the resistors’ thermal noise.
In the equations below, ROUTDAC is DAC output resistance, RIN_DIFF is op-amp input resistance, COUTDAC is DAC output capacitance, and CIN is op-amp input resistance. For the transimpedance circuit, noise gain GN varies with frequency:
Thermal noise per root hertz from RF at the output is:
Output voltage noise density from just the op amp is:
Output noise density from noise current through RF is:
The transimpedance amp’s total output noise density is:
At 100 kHz, total output noise density is 1.75 nV/vHz. For the 100-MHz band above 100 kHz, total output noise is 17.5 µV. For comparison, a 14-bit LSB for 1-V full-scale is 61 µV.
DIFFERENTIAL VOLTAGE CONVERTER NOISE The differential circuit has more output noise. Noise gain GN is a constant 1.91. It’s lower than the expected 2 due to ROB. For noise analysis, treat the sums RINA = R1A + ROA and RINB = R1B + ROB as single resistors.
For each resistor, input-referred thermal noise density is:
Output voltage noise density from just the op amp is VNamp = VN × GN. Output noise density from noise current through each resistor combination is:
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