AD7190BRUZ-REEL Analog Devices Inc, AD7190BRUZ-REEL Datasheet - Page 33

• Manufacturer Part Number: AD7190BRUZ-REEL
• Description: 2ch UltraLow Noise 24Bit SD ADC IC.
• Manufacturer: Analog Devices Inc

Specifications of AD7190BRUZ-REEL

• Design Resources: Precision Weigh Scale Design Using AD7190 with Internal PGA (CN0102)
• Number Of Bits: 24
• Sampling Rate (per Second): 4.8k
• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Converters: 1
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP (0.173", 4.40mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7190 has two differential/four pseudo differential analog
input channels which can be buffered or unbuffered. In buffered
mode (the BUF bit in the configuration 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 at a gain of 1 such that no gain error at the 20-bit level is
introduced.
Table 22. 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 AGND + 250 mV and AV
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 AGND – 50 mV and AV
negative absolute input voltage limit does allow the possibility
of monitoring small true bipolar signals with respect to AGND.
PGA
When the gain stage is enabled, the output from the buffer is
applied to the input of the programmable gain array (PGA).
The presence of the PGA means that signals of small amplitude
can be gained within the AD7190 while still maintaining excel-
lent noise performance. For example, when the gain is set to
128, the rms noise is 8.5 nV, typically, when the output data rate
is 4.7 Hz, which is equivalent to 23 bits of effective resolution or
20.5 bits of noise-free resolution.
The AD7190 can be programmed to have a gain of 1, 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 19.53 mV to 0 V to 2.5 V, and the bipolar
ranges are from ±19.53 mV to ±2.5 V.
R (Ω)
1.4 k
850
300
230
30
DD
Table 22
+ 50 mV. The
DD
– 250 mV. Care
shows the
Rev. B | Page 33 of 40
The analog input range must be limited to (AV
because the PGA requires some headroom. Therefore, if AV
5 V, the maximum analog input that can be applied to the
AD7190 is 0 to 3.75 V/gain in unipolar mode or ±3.75 V/ gain
in bipolar mode.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7190 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 AGND. In pseudo-differential mode, signals are
referenced to AINCOM while in differential mode, signals are
referenced to the negative input of the differential pair. For
example, if AINCOM is 2.5 V and the AD7190 AIN1 analog
input is configured for unipolar mode with a gain of 2, the input
voltage range on the AIN1 pin is 2.5 V to 3.75 V when a 2.5 V
reference is used. If AINCOM is 2.5 V and the AD7190 AIN1
analog input is configured for bipolar mode with a gain of 2, the
analog input range on AIN1 is 1.25 V to 3.75 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
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.
gain is the PGA setting (1 to 128).
N = 24.
CLOCK
The AD7190 includes an internal 4.92 MHz clock on-chip. This
internal clock has a tolerance of ±4%. Either the internal clock
or an external crystal/clock can be used as the clock source to
the AD7190. The clock source is selected using the CLK1 and
CLK0 bits in the mode register. When an external crystal is
used, it must be connected across the MCLK1 and MCLK2
pins. The crystal manufacturer recommends the load capaci-
tances required for the crystal. The MCLK1 and MCLK2 pins of
the AD7190 have a capacitance of 15 pF, typically. If an external
Code = (2
Code = 2
N – 1
N
× AIN × gain)/V
× [(AIN × gain/V
REF
REF
) + 1]
DD
− 1.25 V)/gain
AD7190
DD
=