tiny26 internal ADC amplifier

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Hello,

I see that the tiny26 has a internal gain amplifier for the ADC. Does this amplifier reduce the input current requirements of the S/H?
The datasheet is rather vague on this. According to the datasheet, the input resistance of all AVR ADCs with or without internal amplifier is 100Mohm and they simply neglect to mention the input current requirements of the S/H.

There is also the question regarding the input leakage current. The datasheet does not indicate if the input leakage current (1uA) specified for normal I/O pins is also present for ADC pins.

Assuming the input leakage current is well below 1uA for ADC pins, then perhaps the tiny26 has a nice truly high input impedance ADC which would make it possible to get away without external OP amps.

Any thoughts?

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Hi,

usually the S/H are only some CMOS switches and a capacitor.
I think the datasheet says enough. The most important is the capacitance and the sampletime.
but if this is influenced by the amplifier - I can´t find it.(it depends whether the amp is in front or after the S/H

If you have a high impedance signal source than the S/H capacitor can not be charged fully and therfore errors occur.

If you use DC as an input signal, and place an capacitor of about 10nF at the ADCinput (i recommend it) then the DC input current should be negligible with an input source <10kOhms (datasheet).

Let´s calculate:
1uA x 10kOhms = 10mV. --> would give 4LSB with 2.56VRef and 10Bits.

Klaus
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The amp is in front of the S/H, so it should solve the S/H capacitor input current problems..

Thing is, the datasheet claims a ADC input resistance of 100Mohms, but at the same time it somewhat vaguely indicates that there may be leakage currents of up to 1uA.
These numbers cannot both be true. It takes a much lower input resistance than 100Mohm to generate a current of 1uA.

A 10nF capacitor on a high impedance signal would create a very significant RC filter, blocking out high frequencies.