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VCA810: Input impedance

Olaf Wischhusen
Prodigy 60 points
Part Number: VCA810

Hello,

when simulating the circuit shown in datasheet fig. 36 and connecting the input voltage of 10 mV via a series resistor I get a significant input signal attenuation.

The datasheet states an input impedance of 1 Meg but calculations with my simulation result in an input impedance of only approx. 1.4k.

What can be the reason?

Best regards,

Olaf

  • 0
    Guru 140200 points

    Hi Olaf,

    could be an issue of the Spice model.

    Nevertheless, when you carefully look into the datasheet (figures 30, 34, 35, 41, 43 and 45), the VCA810 is always used with very small source resistances at the inputs. This should ring your alarm bells and hinder you in mounting bigger source resistances to the inputs. Keep in mind that this is no standard OPAmp but a wideband voltage controlled amplifier with a somewhat different input topology. Also see the huge input bias current of -6µA typically. This means that the source resistances at the two inputs of VCA810 should not only be small but also equal. Too high source resistances can also increase the noise and the distortion.

    Kai