ADIS16203
Rev. A | Page 22 of 28
PERIPHERALS
AUXILIARY ADC FUNCTION
The auxiliary ADC function integrates a standard 12-bit ADC
into the ADIS16203 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. 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 32 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 to start conducting. These diodes 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 . Capacitor C2 represents
the ADC sampling capacitor and is typically 16 pF.
C2
C1
R1
DD
D
D
06108-028
Figure 32. 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 less than 1 k. 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 ADIS16203. The DAC output is buffered and fed off-chip
to allow for the control of miscellaneous system-level functions.
Data downloads 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 and, 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 k 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 non-
linear region at the bottom of the transfer function becomes
larger. Larger current demands can significantly limit output
voltage swing.
AUX_DAC Register Definition
Address Default
1
Format Access
0x31, 0x30 0x0000 Binary R/W
1
Default is valid only until the first register write cycle.
The AUX_DAC register controls the DAC function of the
ADIS16203. 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 register has read/write capability.
Table 22. AUX_DAC Bit Descriptions
Bit Description
15:12 Not used
11:0 Data bits
