20-08-2017, 05:39 PM
(20-08-2017, 10:21 AM)Mark Hennessy Wrote: The input travels via a 10M resistance straight to to the IC. Inside the IC, that signal goes straight to an amplifier, but at the input to the amplifier are a bunch of FET switches that are used to connect external resistors to form a potential divider with the 10M, according to the required range. In mV, none of these resistors are engaged, hence the high input impedance. The fact there are two millivolt ranges for both the AN8002 and AN8008 is because of the ability to switch the input amplifier between x1 and x10.
A high value resistor like that in series with the input must give horrible AC frequency response unless they fit compensation caps, a bit like in a scope probe. If the chip has a 10pF input capacitance and the reisistor is 10M the 3dB point will be under 2kHz.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
