Datasheet

AD7949 Data Sheet
Rev. D | Page 18 of 32
Input Configurations
Figure 29 shows the different methods for configuring the analog
inputs with the configuration register, CFG[12:10]. Refer to the
Configuration Register, CFG, section for more details.
The analog inputs can be configured as
• Figure 29A, single-ended referenced to system ground;
CFG[12:10] = 111
2
.
In this configuration, all inputs (IN[7:0]) have a range of
GND to V
REF
.
• Figure 29B, bipolar differential with a common reference
point; COM = V
REF
/2; CFG[12:10] = 010
2
.
Unipolar differential with COM connected to a ground
sense; CFG[12:10] = 110
2
.
In these configurations, all inputs IN[7:0] have a range of
GND to V
REF
.
• Figure 29C, bipolar differential pairs with the negative
input channel referenced to V
REF
/2; CFG[12:10] = 00X
2
.
Unipolar differential pairs with the negative input channel
referenced to a ground sense; CFG[12:10] = 10X
2
.
In these configurations, the positive input channels have
the range of GND to V
REF
. The negative input channels are
senses referred to V
REF
/2 for bipolar pairs, or GND for
unipolar pairs. The positive channel is configured with
CFG[9:7]. If CFG[9:7] is even, then IN0, IN2, IN4, and IN6
are used. If CFG[9:7] is odd, then IN1, IN3, IN5, and IN7
are used (channels with parentheses). For example, for
IN0/IN1 pairs with the positive channel on IN0, CFG[9:7]
= 000
2
. For IN4/IN5 pairs with the positive channel on
IN5, CFG[9:7] = 101
2
.
Note that for the sequencer, detailed in the Channel
Sequencer section, the positive channels are always IN0,
IN2, IN4, and IN6.
• Figure 29D, inputs configured in any of the preceding
combinations (showing that the AD7949 can be configured
dynamically).
GND
COM
CH0+
CH3+
CH1+
CH2+
CH4+
CH5+
CH6+
CH7+
CH0+
CH3+
CH1+
CH2+
CH4+
CH5+
CH6+
CH7+
COM–
GND
COM
IN1
IN0
IN2
IN3
IN4
IN5
IN6
IN7
IN1
IN0
IN2
IN3
IN4
IN5
IN6
IN7
IN1
IN0
IN2
IN3
IN4
IN5
IN6
IN7
IN1
IN0
IN2
IN3
IN4
IN5
IN6
IN7
A—8 CHANNELS,
SINGLE ENDED
B—8 CHANNELS,
COMMON REFERNCE
GND
COM
CH0+ (–)
CH1+ (–)
CH2+ (–)
CH3+ (–)
CH0– (+)
CH1– (+)
CH0+ (–)
CH1+ (–)
CH0– (+)
CH1– (+)
CH2– (+)
CH3– (+)
C—4 CHANNELS,
DIFFERENTIAL
GND
COM
CH2+
CH3+
CH4+
CH5+
D—COMBINATION
COM–
07351-029
Figure 29. Multiplexed Analog Input Configurations
Sequencer
The AD7949 includes a channel sequencer useful for scanning
channels in a repeated fashion. Refer to the Channel Sequencer
section for further details of the sequencer operation.
Source Resistance
When the source impedance of the driving circuit is low, the
AD7949 can be driven directly. Large source impedances
significantly affect the ac performance, especially THD. The dc
performances are less sensitive to the input impedance. The
maximum source impedance depends on the amount of THD
that can be tolerated. The THD degrades as a function of the
source impedance and the maximum input frequency.