ADIS16201PCB datasheet(27/32 Pages) AD

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Data Sheet

ADIS16201

Rev. C | Page 27 of 32

PERIPHERALS

AUXILIARY ADC FUNCTION

The auxiliary ADC function integrates a standard 12-bit ADC

into the ADIS16201 to digitize other system-level analog signals.

The output of the ADC can be monitored through the AUX_ADC

control register, as defined in Table 6 and Table 7. The ADC

consists of a 12-bit successive approximation converter. The

output data is presented in straight binary format, with the full

scale range extending from 0 V to VREF. A high precision, low

drift, factory-calibrated 2.5 V reference is also provided.

Figure 38 shows the equivalent circuit of the analog input

structure 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 would cause these

diodes to become forward-biased and start conducting. They

can handle 10 mA without causing irreversible damage to the

part. 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.

VDD

Figure 38. 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 through the use of an

RC low-pass filter on the relevant analog input pins.

In applications where harmonic distortion and signal-to-noise

ratio are critical, the analog input should 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. The maximum source impedance

depends on the amount of total harmonic distortion (THD)

that can be tolerated.

AUXILIARY DAC FUNCTION

The auxiliary DAC function integrates a standard 12-bit DAC

into the ADIS16201. The DAC output is buffered and fed off-

chip to allow for the control of miscellaneous system-level

functions. Data is downloaded through the writing of two

adjacent data bytes, as defined in its register definition. To

prevent the DAC from transitioning through inadvertent states

during data downloads, a single command is used to

simultaneously latch both data bytes into the DAC after they

have been written into the AUX_DAC control register. This

command is implemented by writing 1 to Bit 2 of the command

control register, which, once received, results in the DAC output

transitioning to the desired state.

The DAC output provides an output range of 0 V to 2.5 V. The

DAC output buffer features a true rail-to-rail output stage. This

means that, unloaded, the output is capable of reaching within

5 mV of ground. Moreover, the DAC’s linearity performance

(when driving a 5 kΩ resistive load to ground) is good through

the full transfer function, except for Code 0 to Code 100.

Linearity degradation near ground is caused by saturation of the

output amplifier. As the output is forced to sink more current,

the nonlinear region at the bottom of the transfer function

becomes larger. Larger current demands can significantly limit

output voltage swing.

AUX_DAC Register Definition

Address

Default1

Format

Access

0x31, 0x30

0x0000

Binary

R/W

Default is valid only until the first register write cycle.

The AUX_DAC register controls the ADIS16201’s DAC function.

The data bits provide a 12-bit binary format number with 0

representing 0 V and 0x0FFFh representing 2.5 V. The data

within this register is volatile and is set to 0s upon reset. This

Table 30. AUX_DAC Bit Descriptions

Bit

Description

15:12

Not used

11:0

Data bits

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