AD7785 Analog Devices, AD7785 Datasheet - Page 25

• Manufacturer Part Number: AD7785
• Description: 3-Channel, Low Noise, Low Power, 20-Bit ?-? ADC with On-Chip In-Amp and Reference
• Manufacturer: Analog Devices

Specifications of AD7785

• Resolution (bits): 20bit
• # Chan: 3
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: ± (Vref/Gain)
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7785 has three differential analog input channels. These
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 gain errors, depending on the output impedance
of the source that is driving the ADC input.
allowable external resistance/capacitance values for unbuffered
mode such that no gain error at the 20-bit level is introduced.
Table 17. External R-C Combination for 20-Bit No Gain Error
C (pF)
50
100
500
1000
5000
The AD7785 can be operated in unbuffered mode only when
the gain equals 1 or 2. At higher gains, the buffer is automati-
cally enabled. The absolute input voltage range in buffered
mode is restricted to a range between GND + 100 mV and
AV
is enabled. The absolute input voltage range when the in-amp is
active is restricted to a range between GND + 300 mV and
AV
so that these limits are not exceeded to avoid degradation in
linearity and noise performance.
The absolute input voltage in unbuffered mode includes the
range between GND – 30 mV and AV
being unbuffered. The negative absolute input voltage limit does
allow the possibility of monitoring small true bipolar signals
with respect to GND.
INSTRUMENTATION AMPLIFIER
Amplifying the analog input signal by a gain of 1 or 2 is
performed digitally within the AD7785. However, when the
gain equals 4 or higher, the output from the buffer is applied
to the input of the on-chip instrumentation amplifier. This low
noise in-amp means that signals of small amplitude can be
gained within the AD7785 while still maintaining excellent
noise performance.
DD
DD
– 100 mV. When the gain is set to 4 or higher, the in-amp
− 1.1 V. Take care in setting up the common-mode voltage
R (Ω)
9 k
6 k
1.5 k
900
200
DD
+ 30 mV as a result of
Table 17
shows the
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For example, when the gain is set to 64, the rms noise is 40 nV
typically, which is equivalent to 20 bits effective resolution or
18.5 bits peak-to-peak resolution.
The AD7785 can be programmed to have a gain of 1, 2, 4, 8, 16,
32, 64, and 128 using Bit G2 to Bit G0 in the configuration
register. Therefore, with an external 2.5 V reference, the
unipolar ranges are from 0 mV to 20 mV to 0 V to 2.5 V while
the bipolar ranges are from ±20 mV to ±2.5 V. When the
in-amp is active (gain ≥ 4), the common-mode voltage (AIN(+)
+ AIN(–))/2 must be greater than or equal to 0.5 V.
If the AD7785 is operated with an external reference that has a
value equal to AV
90% of V
operation.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7785 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
and a gain of 1, 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 configura-
tion 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 00000 hex, a midscale voltage
resulting in a code of 80000, and a full-scale input voltage
resulting in a code of FFFFF. 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 00000 hex, a zero differential input voltage resulting
in a code of 80000 hex, and a positive full-scale input voltage
resulting in a code of FFFFF hex. The output code for any
analog input voltage can be represented as
where:
AIN is the analog input voltage.
GAIN is the in-amp setting (1 to 128).
N = 20.
Code = 2
Code = (2
REF
/gain when the in-amp is active, for correct
N – 1
N
× AIN × GAIN)/V
× [(AIN × GAIN /V
DD
, the analog input signal must be limited to
REF
REF
) + 1]
AD7785