ADIS16204

Rev. 0 | Page 11 of 24

AUXILIARY ADC FUNCTION

The auxiliary ADC function integrates a standard 12-bit ADC

into the ADIS16204 to digitize other system-level analog sig-

nals. The output of the ADC can be monitored through the

AUX_ADC control register, as defined in Table 6. The ADC is

a 12-bit successive approximation converter. The output data

is presented in straight binary format with the full-scale range

calibrated 2.5 V reference is also provided.

Figure 20 shows the equivalent circuit of the analog input struc-

ture of the ADC. The input capacitor (C1) is typically 4 pF and

can be attributed to parasitic package capacitance. The two diodes

provide ESD protection for the analog input. Care must be

taken to ensure that the analog input signals never exceed the

supply rails by more than 300 mV. This causes the diodes to

become forward-biased and to start conducting. The diodes

can handle 10 mA without causing irreversible damage. The

resistor is a lumped component that represents the on resistance

of the switches. The value of this resistance is typically 100 Ω.

Capacitor C2 represents the ADC sampling capacitor and is

typically 16 pF.

C2

C1

R1

VDD

D

D

Figure 20. Equivalent Analog Input Circuit

Conversion Phase: Switch Open

Track Phase: Switch Closed

For ac applications, removing high frequency components from

the analog input signal is recommended by the use of a low-pass

filter on the analog input pin.

In applications where harmonic distortion and signal-to-noise

ratio are critical, the analog input must be driven from a low

impedance source. Large source impedances significantly affect

the ac performance of the ADC. This can necessitate the use of

an input buffer amplifier. When no input amplifier is used to drive

the analog input, the source impedance should be limited to

values lower than 1 kΩ.