AD7949 Analog Devices, AD7949 Datasheet - Page 17

• Manufacturer Part Number: AD7949
• Description: 14-Bit, 8-Channel, 250 kSPS PulSAR ADC
• Manufacturer: Analog Devices

Specifications of AD7949

• Resolution (bits): 14bit
• # Chan: 8
• Sample Rate: 250kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Uni
• Adc Architecture: SAR
• Pkg Type: CSP

Data Sheet
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
In this configuration, all inputs (IN[7:0]) have a range of
GND to V
Figure 29B, bipolar differential with a common reference
point; COM = V
Unipolar differential with COM connected to a ground
sense; CFG[12:10] = 110
In these configurations, all inputs IN[7:0] have a range of
GND to V
Figure 29C, bipolar differential pairs with the negative
input channel referenced to V
Unipolar differential pairs with the negative input channel
referenced to a ground sense; CFG[12:10] = 10X
In these configurations, the positive input channels have
the range of GND to V
senses referred to V
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
IN5, CFG[9:7] = 101
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).
2
. For IN4/IN5 pairs with the positive channel on
REF
REF
.
.
REF
2
.
/2; CFG[12:10] = 010
REF
2
.
/2 for bipolar pairs, or GND for
REF
2
.
. The negative input channels are
REF
/2; CFG[12:10] = 00X
2
.
2
.
2
.
Rev. C | Page 17 of 32
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.
CH0+ (–)
CH0– (+)
CH1+ (–)
CH1– (+)
CH2+ (–)
CH2– (+)
CH3+ (–)
CH3– (+)
CH0+
CH1+
CH2+
CH3+
CH4+
CH5+
CH6+
CH7+
Figure 29. Multiplexed Analog Input Configurations
A—8 CHANNELS,
C—4 CHANNELS,
SINGLE ENDED
DIFFERENTIAL
COM
GND
COM
GND
IN0
IN1
IN2
IN3
IN4
IN5
IN6
IN7
IN0
IN1
IN2
IN3
IN4
IN5
IN6
IN7
CH0+ (–)
CH0– (+)
CH1+ (–)
CH1– (+)
COM–
COM–
CH2+
CH3+
CH4+
CH5+
CH1+
CH2+
CH3+
CH4+
CH5+
CH0+
CH6+
CH7+
COMMON REFERNCE
D—COMBINATION
B—8 CHANNELS,
IN2
IN3
IN5
COM
IN0
IN1
IN4
IN6
IN7
COM
GND
GND
IN2
IN3
IN5
IN7
IN0
IN1
IN4
IN6
AD7949