There is one other factor that relates to the impedance of the input path, and that is acquisiton time. Normally they try to make A/D converters acquire the voltage as fast as possible. Otherwise the application will be burdened by unreasonable delay times. But if your application does not require very fast response, you can trade off that input path impedance with acquisition time.I still don't understand how the accuracy is dependent of the impedance in the input path .
An A/D conversion is composed of two distinct phases. The first phase is the acquisition time. During that time the SW1 in your circuit is closed and the input capacitance can charge up to be equal to the signal voltage being sampled. The second phase is the actual conversion to digital in which SW1 is opened and the converter does whatever it needs to do (successive approximation or dual slope). Since the first phase of the process is normally time-limited, the impedance of the input path determines how completely the capacitor charges to the ultimate value. In some A/D converters this acquisition time is fixed. But in others it is programmable, often starting when the application software sets the input channel (assuming there is a multiplexor on the input). If yours is programmable, or if you never switch channels, there is a good chance you can decrease the importance of the impedance of the input path by providing a long enough acquisition phase.
The MCP3421 uses a switched-capacitor input stage
using a 3.2 pF sampling capacitor. This capacitor is
switched (charged and discharged) at a rate of the
sampling frequency that is generated by the on-board
clock.
Since the sampling capacitor is only switching to the
input pins during a conversion process, the above input
impedance is only valid during conversion periods. In a
low power standby mode, the above impedance is not
presented at the input pins. Therefore, only a leakage
current due to ESD diode is presented at the input pins.
The conversion accuracy can be affected by the input
signal source impedance when any external circuit is
connected to the input pins. The source impedance
adds to the internal impedance and directly affects the
time required to charge the internal sampling capacitor.
Therefore, a large input source impedance connected
to the input pins can increase the system performance
errors such as offset, gain, and integral nonlinearity
(INL) errors. Ideally, the input source impedance
should be zero. This can be achievable by using an
operational amplifier with a closed-loop output
impedance of tens of ohms.
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