Figure 3. Digital Pot ideal transfer function.
Inspection of Figure 4 yields the dc transfer function for VOUT/VIN: (4) VOUT/VIN = (R3 + R2bottom)/(R1 + R2 + R3), where R2 = R2top + R2bottom 
Figure 4. Typical Digital Poteniometer circuit configuration with new model for digital potentiometer.
Next, let's make some assumptions:
AssumptionsAssume R2 = 10kΩ (a common digital pot resistance value), and assume we want to attenuate the incoming signal to some arbitrary level, say 70% ±5% of its input value (i.e. from 65% to 75% of its input value).
Then using equations (1)–(4), we can see that an adjustment range of 65% to 75%, with a nominal (mid-scale setting) of 70% occurs for: (5) R1 = 24.9kΩ and R3 = 64.9kΩ. Bandwidth of Typical Applications CircuitUsing the resistance values of Equation (5), and assuming Cwiper = 10pF, we get the bandwidths as listed in Table 1. Actual wiper capacitance can vary from 3pF to over 80pF, and is function of wiper resistance, the number of steps, the IC process used, and the pot architecture used, among other things. 3pF–10pF of capacitance is fairly representative for 3V to 5V, 10kΩ pots, with 32 to 256 steps.
Note that the analysis in this article assumes that it is solely the wiper capacitance in parallel with the pot resistance, which sets the bandwidth. This is valid for relatively straightforward implementations of digital pots, but the bandwidth can be limited even more, if more complicated pot implementations are used. That said, however, the discussions below src="/data/attachment/portal/201007/ET33815201007230417395.gif">
Figure 5. Using src="/data/attachment/portal/201007/ET33815201007230417396.gif">
本人用IRS2110 HO 端始输出奇怪的波形本人买的是IRS2110 （和IR2110不一样吗），LO一直可以输出，HO 输出一个很乱的波。于是我把自举二极管拿了。发现VS脚还会输出一个矩形不规则波（并没有给VS脚通电）
怎么个
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