EVAL-AD7709EB Analog Devices Inc, EVAL-AD7709EB Datasheet - Page 23

• Manufacturer Part Number: EVAL-AD7709EB
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7709EB

• Lead Free Status / Rohs Status: Compliant

REV. A
Programmable Gain Amplifier
The output from the buffer on the ADC is applied to the input
of the on-chip programmable gain amplifier (PGA). The PGA
can be programmed through eight different unipolar and bipolar
ranges. The PGA gain range is programmed via the range bits
in the Configuration Register. With an external 2.5 V reference
applied, the unipolar ranges are 0 mV to 20 mV, 0 mV to 40 mV,
0 mV to 80 mV, 0 mV to 160 mV, 0 mV to 320 mV, 0 mV to
640 mV, 0 V to 1.28 V, and 0 to 2.56 V, while bipolar ranges
are ± 20 mV, ± 40 mV, ± 80 mV, ± 160 mV, ± 320 mV, ± 640
mV, ± 1.28 V, and ± 2.56 V. These are the ranges that should
appear at the input to the on-chip PGA.
Typical matching across ranges is shown in Figure 15. Here, the
ADC is configured in fully differential, bipolar mode with an
external 2.5 V reference, while an analog input voltage of just
greater than 19 mV is forced on its analog inputs. The ADC
continuously converts the dc voltage at an update rate of 5.35 Hz,
i.e., SF = FFh. A total of 800 conversion results are gathered.
The first 100 results gathered with the ADC operating in the
± 20 mV. The ADC range is then switched to ± 40 mV and 100
more results are gathered, and so on, until the last 100 samples
are gathered with the ADC configured in the ± 2.5 V range. From
Figure 15, the variation in the sample mean through each range,
i.e., the range matching, is seen to be on the order of 2 µV.
Bipolar/Unipolar Configuration
The analog inputs on the AD7709 can accept either unipolar or
bipolar input voltage ranges. Bipolar input ranges do not imply that
the part can handle negative voltages with respect to system
GND. Unipolar and bipolar signals on the AIN(+) input on
the ADC are referenced to the voltage on the respective AIN(–)
input. AIN(+) and AIN(–) refer to the signals seen by the
modulator that come from the output of the multiplexer, as shown
in Figures 16 and 17.
SAMPLE COUNT
ADC RANGE
19.372
19.371
19.370
19.369
19.368
19.367
19.366
19.365
19.364
Figure 15. ADC Range Matching
0
100
200
300
400
500
600
700
800
–23–
For example, if AIN(–) is 2.5 V and the ADC is configured for
an analog input range of 0 mV to 20 mV, the input voltage range
on the AIN(+) input is 2.5 V to 2.52 V. If AIN(–) is 2.5 V and
the AD7709 is configured for an analog input range of ± 1.28 V,
the analog input range on the AIN(+) input is 1.22 V to 3.78 V
(i.e., 2.5 V ± 1.28 V). Bipolar or unipolar options are chosen by
programming the UNI bit in the Configuration Register. This
programs the ADC for either unipolar or bipolar operation.
Programming for either unipolar or bipolar operation does not
change any of the input signal conditioning; it simply changes
the data output coding.
Data Output Coding
When the ADC is configured for unipolar operation, the output
coding is natural (straight) binary with a zero differential input
voltage resulting in a code of 000 . . . 000, a midscale voltage
resulting in a code of 100 . . . 000, and a full-scale input voltage
resulting in a code of 111 . . . 111. The output code for any analog
input voltage on the ADC can be represented as follows:
where:
FULLY DIFFERENTIAL
FULLY DIFFERENTIAL
AINCOM
AIN is the analog input voltage.
GAIN is the PGA gain, i.e., 1 on the 2.56 V range and 128 on
the 20 mV range.
N = 16.
AIN1
AIN2
AIN3
AIN4
AIN1
AIN2
AIN3
AIN4
Figure 17. Pseudo-Differential Mode of Operation
Figure 16. Fully Differential Mode of Operation
AIN4/AINCOM
Code
AIN3/AINCOM
PSEUDO-DIFFERENTIAL
PSEUDO-DIFFERENTIAL
AIN2/AINCOM
=
(
AIN1/AINCOM
AIN
INPUT
INPUT
(
1 024
.
×
AIN1
AIN2
AIN3
AIN4
GAIN
×
V
MUX
REF
AIN1
AIN2
AIN3
AIN4
AINCOM
×
)
2
N
MUX
)
ADC CHANNEL
AIN(+)
AIN(–)
ADC CHANNEL
AD7709
AIN(+)
AIN(–)