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# 跨阻再次罢工：利用MDACs实现电流电压转换

The second subnetwork is the amplifier-feedback network. To maintain precision, most MDACs have a feedback resistor on

-chip. The feedback capacitor, CF, is discrete.

Finally, op amps have a range of specifications, but only a few affect the MDAC circuit’s stability: unity-gain bandwidth, fU; input differential capacitance, CDIF; and common-mode capacitance, CCM.

In this system, total capacitance at the amplifier input is equal to CIN="CD"+CDIF+CCM. In Figure 1b and Figure 1c, the closed-loop zero is equal to f1=1/(2π(CIN+CF)(RD||RF)). The closed-loop pole is equal to f2=1/(2πCFRF).

You ensure system stability if the rate of closure between the open- and closed-loop-gain curve equals 20 dB/decade. To do so, select an amplifier with unity-gain bandwidth of less than f1 or greater than f2.

It is easy to design a stable circuit if f1 is higher than the amplifier’s bandwidth:

Alternatively, if f2 is lower than the intersection of the open- and the closed-loop-gain curve, use:

CF≤–CIN+1/(2/π(RF||RD)fU).

Use these calculated values of feedback capacitance as starting points for your test circuit. Circuit parasitics, device-manufacturing variations, and other factors can encourage you to modify the feedback-capacitor value.

Stabilizing the MDAC’s analog signal is critical. However, also consider amplifier noise, input bias current, offset voltage, MDAC resolution, and glitch energy.

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