EVAL-AD7797EBZ Analog Devices Inc, EVAL-AD7797EBZ Datasheet - Page 20

• Manufacturer Part Number: EVAL-AD7797EBZ
• Description: BOARD EVALUATION FOR AD7797
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7797EBZ

• Number Of Adc's: 1
• Number Of Bits: 24
• Sampling Rate (per Second): 123
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF/128
• Power (typ) @ Conditions: 1.6mW @ 123SPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7797
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7796/AD7797
CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7796/AD7797 have one differential analog input
channel. The input channel feeds into a high impedance input
stage of the amplifier. Therefore, the input can tolerate signifi-
cant source impedances and is tailored for direct connection to
external resistive-type sensors such as strain gages.
The absolute input voltage range is restricted to a range between
GND + 300 mV and AV
setting up the common-mode voltage to avoid exceeding these
limits. Otherwise, there is degradation in linearity and noise
performance.
This low noise in-amp means that signals of small amplitude
can be gained within the AD7796/AD7797 while still maintain-
ing excellent noise performance. The amplifier is configured to
have a gain of 128. Therefore, with an external 2.5 V reference,
the unipolar range is 0 mV to 20 mV while the bipolar range is
±20 mV. The common-mode voltage ((AIN(+) + AIN(–))/2
must be ≥ 0.5 V.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7796/AD7797 can accept either
unipolar or bipolar input voltage ranges. A bipolar input range
does not imply that the part can tolerate negative voltages with
respect to system GND. Unipolar and bipolar signals on the
AIN(+) input are referenced to the voltage on the AIN(–) input.
For example, if AIN(−) is 2.5 V and the ADC is configured for
unipolar mode, the input voltage range on the AIN(+) pin is
2.5 V to 2.02 V.
If the ADC is configured for bipolar mode, the analog input
range on the AIN(+) input is 2.48 V to 2.52 V. The bipolar/
unipolar option is chosen by programming the U/ B bit in the
configuration register.
DATA OUTPUT CODING
When the ADC is configured for unipolar operation, the output
code is natural (straight) binary with a zero differential input
voltage resulting in a code of 00...00, 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 can be represented as
Code = (2
N
× AIN × 128)/V
DD
− 1.1 V. Care must be taken in
REF
Rev. A | Page 20 of 24
When the ADC is configured for bipolar operation, the output
code is offset binary with a negative full-scale voltage resulting
in a code of 000...000, a zero differential input voltage resulting
in a code of 100...000, and a positive full-scale input voltage
resulting in a code of 111...111. The output code for any analog
input voltage can be represented as
where:
AIN is the analog input voltage
N = 16/24 for the AD7796/AD7797.
REFERENCE
The AD7796/AD7797 have a fully differential input capability
for the channel. The common-mode range for these differential
inputs is GND to AV
therefore, excessive R-C source impedances introduce gain
errors. The reference voltage REFIN (REFIN(+) − REFIN(−)) is
2.5 V nominal, but the AD7796/AD7797 are functional with
reference voltages 0.1 V to AV
excitation (voltage or current) for the transducer on the analog
input also drives the reference voltage for the part, the effect of
the low frequency noise in the excitation source is removed
because the application is ratiometric. If the AD7796/AD7797
are used in a nonratiometric application, a low noise reference
should be used.
Recommended 2.5 V reference voltage sources for the AD7796/
AD7797 include the ADR381 and ADR391, which are low
noise, low power references. Also note that the reference
inputs provide a high impedance, dynamic load. Because
the input impedance of each reference input is dynamic,
resistor/capacitor combinations on these inputs can cause
dc gain errors, depending on the output impedance of the
source that is driving the reference inputs.
Reference voltage sources such as those recommended above
(the ADR391, for example) typically have low output
impedances and are, therefore, tolerant to decoupling capacitors
on REFIN(+) without introducing gain errors in the system.
Deriving the reference input voltage across an external resistor
means that the reference input sees a significant external source
impedance. External decoupling on the REFIN pins is not
recommended in this type of circuit configuration.
Code = 2
N – 1
× [(AIN × 128 /V
DD
. The reference input is unbuffered;
DD
. In applications where the
REF
) + 1]