EVAL-AD7787EB Analog Devices Inc, EVAL-AD7787EB Datasheet - Page 18

• Manufacturer Part Number: EVAL-AD7787EB
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7787EB

• Lead Free Status / Rohs Status: Not Compliant

AD7787
CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7787 has two analog input channels that are connected
to the on-chip buffer amplifier when the device is operated in
buffered mode and directly to the modulator when the device is
operated in unbuffered mode. In buffered mode (the BUF bit in
the mode register is set to 1), the input channel feeds into a high
impedance input stage of the buffer amplifier. Therefore, the
input can tolerate significant source impedances and is tailored
for direct connection to external resistive-type sensors, such as
strain gauges or resistance temperature detectors (RTDs).
When BUF = 0, the part is operated in unbuffered mode.
This results in a higher analog input current. Note that this
unbuffered input path provides a dynamic load to the driving
source. Therefore, resistor/capacitor combinations on the input
pins can cause dc gain errors, depending on the output
impedance of the source that is driving the ADC input. Table 15
shows the allowable external resistance/capacitance values for
the unbuffered mode such that no gain error at the 20-bit level
is introduced.
Table 15. External R-C Combination for No 20-Bit Gain Error
C (pF)
50
100
500
1000
5000
The absolute input voltage range in buffered mode is restricted
to a range between GND + 100 mV and V
must be taken in setting up the common-mode voltage so that
these limits are not exceeded. Otherwise, there is degradation in
linearity and noise performance.
The absolute input voltage in unbuffered mode includes the
range between GND − 100 mV and V
being unbuffered. The negative absolute input voltage limit does
allow the possibility of monitoring small true bipolar signals
with respect to GND.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7787 can accept either unipolar or
bipolar input voltage ranges. Unipolar and bipolar signals on the
AIN1(+) input are referenced to the voltage on the AIN(−)
input. For example, if AIN1(−) is 2.5 V and the ADC is
configured for unipolar mode, the input voltage range on the
AIN1(+) pin is 2.5 V to 5 V when REFIN = 2.5 V. If the ADC is
configured for bipolar mode, the analog input range on the
AIN1(+) input is 0 V to 5 V.
R (Ω)
16.7 k
9.6 k
2.2 k
1.1 k
160
DD
+ 30 mV resulting from
DD
− 100 mV. Care
Rev. 0 | Page 18 of 20
The voltage on AIN2 is referenced to GND. Therefore, when
bipolar mode is selected and the part is operated in unbuffered
mode, the voltage on AIN2 can vary from GND − 100 mV to
+2.5 V. In unipolar mode, the voltage on AIN2 can vary from
programming the U/ B bit in the mode 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
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 and N = 24.
REFERENCE INPUT
The AD7787 has a single-ended reference that is 2.5 V nominal,
but the AD7787 is functional with reference voltages from 0.1 V
to V
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 AD7787 is used in a nonratiometric
application, a low noise reference should be used.
Recommended 2.5 V reference voltage sources for the AD7787
include the ADR381 and ADR391, which are low noise, low
power references. In a system that operates from a 2.5 V power
supply, the reference voltage source requires some headroom. In
this case, a 2.048 V reference, such as the ADR380 or ADR390,
can be used, requiring only 300 mV of headroom. Also note that
the reference input provides a high impedance, dynamic load.
Because the input impedance of the reference input is dynamic,
resistor/capacitor combinations on this input can cause dc gain
errors, depending on the output impedance of the source
driving the reference inputs.
0 V to 2.5 V. The bipolar/unipolar option is chosen by
DD
Code
Code
. In applications where the excitation (voltage or current)
=
=
2
2
N
N
−
×
1
×
(
AIN
[
(
AIN
/
V
/
REF
V
REF
)
)
+
1
]