AD7789 Analog Devices, AD7789 Datasheet - Page 17

• Manufacturer Part Number: AD7789
• Description: 24-Bit, Single-Channel, Ultra Low Power, Sigma Delta A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7789

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7788/AD7789 have one differential analog input
channel that is connected to the modulator, thus, the input is
unbuffered. 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 13 shows the allowable
external resistance/capacitance values such that no gain error at
the 16-bit level is introduced (AD7788). Table 14 shows the
allowable external resistance/capacitance values such that no
gain error at the 20-bit level is introduced (AD7789).
Table 13. External R-C Combination for No 16-Bit Gain
Error (AD7788)
C (pF)
50
100
500
1000
5000
Table 14. External R-C Combination for No 20-Bit Gain
Error (AD7789)
C (pF)
50
100
500
1000
5000
The absolute input voltage includes the range between GND −
30 mV and V
limit does allow the possibility of monitoring small true bipolar
signals with respect to GND.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the devices can accept either unipolar or
bipolar input voltage ranges. A bipolar input range does not
imply that the parts can tolerate large 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 5 V. If the ADC is configured for bipolar mode, the
analog input range on the AIN(+) input is 0 V to 5 V. The
bipolar/unipolar option is chosen by programming the U/ B bit
in the mode register.
DD
+ 30 mV. The negative absolute input voltage
R (Ω)
16.7 k
9.6 k
2.2 k
1.1 k
160
R (Ω)
22.8 k
13.1 k
3.3 k
1.8 k
360
Rev. B | Page 17 of 20
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 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 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.
N = 16 for the AD7788, 24 for the AD7789.
REFERENCE INPUT
The AD7788/AD7789 have a fully differential input capability
for the channel. The common-mode range for these differential
inputs is from GND to V
and, therefore, excessive R-C source impedances introduce gain
errors. The reference voltage REFIN [REFIN(+) − REFIN(−)] is
2.5 V nominal, but the AD7788/AD7789 are functional with
reference voltages from 0.1 V to V
excitation (voltage or current) for the transducer on the analog
input also drives the reference voltage for the parts, the effect of
the low frequency noise in the excitation source is removed
because the application is ratiometric. If the AD7788/AD7789
are used in a nonratiometric application, a low noise reference
should be used.
Recommended 2.5 V reference voltage sources for the AD7788/
AD7789 include the ADR381 and ADR391, because they are
low noise, low power references. If the analog circuitry uses 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. Again, these are low power, low noise
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.
Code = 2
Code = 2
N – 1
N
× (AIN/V
× [(AIN/V
REF
DD
. The reference input is unbuffered
)
REF
) + 1]
DD
. In applications where the
AD7788/AD7789