MCP3553 ADC und sein Vref-Eingang

3.1 Voltage Reference (VREF)

The MCP3550/1/3 devices accept single-ended reference

voltages from 0.1V to VDD. Since the converter

output noise is dominated by thermal noise, which is

independent of the reference voltage, the output noise

is not significantly improved by diminishing the reference

voltage at the VREF input pin. A reduced voltage

reference will significantly improve the INL performance

(see Figure 2-4); the INL max error is

proportional to VREF²

4.5 Voltage Reference Input Pin

The MCP3550/1/3 devices accept a single-ended

external reference voltage, to be connected on the

VREF input pin. Internally, the reference voltage for the

ADC is a differential voltage with the non-inverting input

connected to the VREF pin and the inverting input

connected to the VSS pin. The value of the reference

voltage is VREF - VSS and the common mode of the

reference is always (VREF - VSS)/2.

The MCP3550/1/3 devices accept a single-ended

reference voltage from 0.1V to VDD. The converter

output noise is dominated by thermal noise that is

independent of the reference voltage. Therefore, the

output noise is not significantly improved by lowering

the reference voltage at the VREF input pin. However, a

reduced reference voltage will significantly improve the

INL performance since the INL max error is

proportional to VREF

2 (see Figure 2-4).

The charge and discharge of the input capacitor create

dynamic input currents at the VREF input pin inversely

proportional to the sampling capacitor, which is a function

of the input reference voltage. The typical value of

the single-ended input impedance is 2.4 MΩ, with

VDD = VREF = 5V. The DC leakage current caused by

the ESD input diodes, though on the order of 1 nA

typically, can cause additional gain error proportional to

the source resistance at the VREF pin.