Datasheet

AD7794/AD7795
Rev. D | Page 22 of 36
CONFIGURATION REGISTER
RS2, RS1, RS0 = 0, 1, 0; Power-On/Reset = 0x0710
The configuration register is a 16-bit read/write register that is
used to configure the ADC for unipolar or bipolar mode, enable
or disable the buffer, enable or disable the burnout currents,
select the gain, and select the analog input channel.
Table 20 outlines the bit designations for the filter register.
CON0 through CON15 indicate the bit locations. CON denotes
that the bits are in the configuration register. CON15 is the first
bit of the data stream. The number in parentheses indicates the
power-on/reset default status of that bit.
CON15 CON14 CON13 CON12 CON11 CON10 CON9 CON8
VBIAS1(0) VBIAS0(0) BO(0)
U/
B(0)
BOOST(0) G2(1) G1(1) G0(1)
CON7 CON6 CON5 CON4 CON3 CON2 CON1 CON0
REFSEL1(0) REFSEL0(0) REF_DET(0) BUF(1) CH3(0) CH2(0) CH1(0) CH0(0)
Table 20. Configuration Register Bit Designations
Bit No. Mnemonic Description
CON15 to
CON14
VBIAS1 to VBIAS0
Bias Voltage Generator Enable. The negative terminal of the analog inputs can be biased up to AV
DD
/2.
These bits are used in conjunction with the BOOST bit.
VBIAS1 VBIAS0 Bias Voltage
0 0 Bias voltage generator disabled
0 1 Bias voltage generator connected to AIN1(−)
1 0 Bias voltage generator connected to AIN2(−)
1 1 Bias voltage generator connected to AIN3(−)
CON13 BO
Burnout Current Enable Bit. This bit must be programmed with a Logic 0 for correct operation. 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 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 positive full-scale differential input results in
an output code of 0xFFFFFF.
CON11 BOOST
This bit is used in conjunction with the VBIAS1 and VBIAS0 bits. When set, the current consumed by the
bias voltage generator is increased, which reduces its power-up time.
CON10 to
CON8
G2 to G0
Gain Select Bits.
Written 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
REFSEL1/REFSEL0
Reference Select Bits.
The reference source for the ADC is selected using these bits.
REFSEL1 REFSEL0 Reference Source
0 0 External reference applied between REFIN1(+) and REFIN1(−)
0 1 External reference applied between REFIN2(+) and REFIN2(−)
1 0 Internal 1.17 V reference
1 1 Reserved