AD7798/AD7799 Data Sheet
Rev. B | Page 16 of 28
CONFIGURATION REGISTER
RS2, RS1, RS0 = 0, 1, 0; Power-On/Reset = 0x0710
The configuration register is a 16-bit register from which data can be read or to which data can be written. This register is used to
configure the ADC for unipolar or bipolar mode, to enable or disable the buffer, to enable or disable the burnout currents, to select the
gain, and to select the analog input channel. Table 15 outlines the bit designations for the filter register. CON0 through CON15 indicate
the bit locations, with CON denoting that the bits are in the configuration register. CON15 denotes the first bit of the data stream. The
number in parentheses indicates the power-on/reset default status of the bit.
CON15 CON14 CON13 CON12 CON11 CON10 CON9 CON8
0(0) 0(0) BO(0) U/
B
(0) 0(0) G2(1) G1(1) G0(1)
CON7 CON6 CON5 CON4 CON3 CON2 CON1 CON0
0(0) 0(0) REF_DET(0) BUF(1) 0(0) CH2(0) CH1(0) CH0(0)
Table 15. Configuration Register Bit Designations
Bit Location Bit Name Description
These bits must be programmed with a Logic 0 for correct operation.
CON13 BO Burnout Current Enable Bit. When this bit is set to 1 by the user, the 100 nA current sources in the signal path
are enabled. When BO = 0, the burnout currents are disabled. The burnout currents can be enabled only when
the buffer or in-amp is active.
CON12
U/
B
Unipolar/Bipolar Bit. Set by the user to enable unipolar coding, that is, zero differential input results in
0x000000 output, and a full-scale differential input results in 0xFFFFFF output. Cleared by the user to enable
bipolar coding. Negative full-scale differential input results in an output code of 0x000000, zero differential
input results in an output code of 0x800000, and a positive full-scale differential input results in an output
code of 0xFFFFFF.
CON11 0 This bit must be programmed with a Logic 0 for correct operation.
CON10 to CON8 G2 to G0 Gain Select Bits. Written to by the user to select the ADC input range as follows:
G2 G1 G0 Gain ADC Input Range (2.5 V Reference)
0 0 0 1 (in-amp not used) 2.5 V
0 0 1 2 (in-amp not used) 1.25 V
0 1 0 4 625 mV
0 1 1 8 312.5 mV
1 0 0 16 156.2 mV
1 0 1 32 78.125 mV
1 1 0 64 39.06 mV
1 1 1 128 19.53 mV
CON7 to CON6 0 These bits must be programmed with a Logic 0 for correct operation.
CON5 REF_DET Enables the reference detect function. When REF_DET is set, the NOREF bit in the status register indicates
when the external reference being used by the ADC is open circuit or less than 0.5 V. When cleared, the
reference detect function is disabled.
CON4 BUF Configures the ADC for buffered or unbuffered modes. If BUF is cleared, the ADC operates in unbuffered
mode, lowering the power consumption of the device. If BUF is set, the ADC operates in buffered mode,
allowing the user to place source impedances on the front end without contributing gain errors to the system.
The buffer can be disabled when the gain equals 1 or 2. For higher gains, the buffer is automatically enabled.
With the buffer disabled, the voltage on the analog input pins can range from 30 mV below GND to 30 mV
above AV
DD
. When the buffer is enabled, it requires some headroom; therefore, the voltage on any input pin
must be limited to 100 mV within the power supply rails.
CON3 0 This bit must be programmed with a Logic 0 for correct operation.
CON2 to CON0 CH2 to CH0 Channel Select Bits. Written to by the user to select the active analog input channel to the ADC as follows:
CH2 CH1 CH0 Channel Calibration Pair
0 0 0 AIN1(+) – AIN1(–) 0
0 0 1 AIN2(+) – AIN2(–) 1
0 1 0 AIN3(+) – AIN3(–) 2
0 1 1 AIN1(–) – AIN1(–) 0
1 0 0 Reserved
1 0 1 Reserved
1 1 0 Reserved
1 1 1 AV
DD
monitor Automatically selects gain = 1/6 and internal
reference = 1.17 V
