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来源:EDN/作者:Bonnie Baker?? 2007年07月20日 ?? 收藏0

  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:


  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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