AD7799 Analog Devices, AD7799 Datasheet - Page 22

• Manufacturer Part Number: AD7799
• Description: 3-Channel, Low Noise, Low Power, 24-Bit, Sigma Delta ADC with On-Chip In-Amp
• Manufacturer: Analog Devices

Specifications of AD7799

• Resolution (bits): 24bit
• # Chan: 3
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

AD7798/AD7799
CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7798/AD7799 each have three differential analog input
channels. These are connected to the on-chip buffer amplifier
when the devices are operated in buffered mode, and directly to
the modulator when the devices are 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 gages or resistance
temperature detectors (RTDs).
When BUF = 0, the parts are 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. Table
allowable external resistance/capacitance values for unbuffered
mode such that no gain error at the 20-bit level is introduced.
Table
Unbuffered Mode (Without 20-Bit Gain Error)
Capacitance (pF)
50
100
500
1000
5000
The AD7798/AD7799 can be operated in unbuffered mode only
when the gain equals 1 or 2. At higher gains, the buffer is auto-
matically 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
mode voltage so that these limits are not exceeded; otherwise,
linearity and noise performance degrade.
The absolute input voltage in unbuffered mode includes the
range between GND − 30 mV and AV
being unbuffered. The negative absolute input voltage limit
allows the possibility of monitoring small true bipolar signals
with respect to GND.
DD
DD
– 100 mV. When the gain is set to 4 or higher, the in-amp
− 1.1 V. Care must be taken in setting up the common-
17
. External Resistance/Capacitance Combination for
Resistance (Ω)
9 k
6 k
1.5 k
900
200
DD
+ 30 mV as a result of
17
shows the
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INSTRUMENTATION AMPLIFIER
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 AD7798/AD7799 while still maintaining
excellent noise performance. For example, when the gain is set
to 64 and the update rate equals 4.17 Hz, the rms noise is 27 nV
typically for the AD7799, which is equivalent to 25.5 bits effective
resolution, or 20 bits peak-to-peak resolution when V
The AD7798/AD7799 can be programmed to have a gain of 1, 2,
4, 8, 16, 32, 64, or 128 using Bit G2 to Bit G0 in the configuration
register. Therefore, with a 2.5 V reference, the unipolar ranges are
from (0 mV to 19.53 mV) to (0 V to 2.5 V), and the bipolar
ranges are from ±19.53 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 AD7798/AD7799 operate with a reference that has a value
equal to AV
V
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7798/AD7799 can accept either
unipolar or bipolar input voltage ranges. A bipolar input range
does not imply that the parts 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 configuration register.
REF
/gain when the in-amp is active for correct operation.
DD
, the analog input signal must be limited to 90% of
REF
= 5 V.