CN0326 Datasheet, PDF (3/7 Pages) Analog Devices – Devices Connected

Circuit Note
For 200 fA typical input bias current, the offset error is 0.2 mV
(0.0037 pH) for a pH probe that has 1 GΩ series resistance at
25^oC. Even at the maximum input bias current of 1 pA, the
error is only 1 mV.
The cut-off frequency of the 10 kΩ/1 µF low pass noise filter for
the buffer amplifier output is given by f= 1/2πRC, or 16 Hz.
Guarding, shielding, high insulation resistance standoffs, and
other such standard picoamp methods must be used to minimize
leakage at the high impedance input of the AD8603 buffer.
ADC Channel 1 Configuration, pH sensor
This stage involves measuring the small voltage generated by
the pH electrode. Table 1 shows the specifications of a typical
pH probe. Based on the Nernst equation, the full range voltage
from the probe can range from ±414 mV (±59.14 mV/pH) at
25°C to ±490 mV (±70 mV/pH) at 80°C.
Table 1. Specifications of a Typical pH Probe
Measurement Range pH 0 to pH 14
pH at zero voltage
pH 7.00 ± 0.25
Accuracy
pH 0.05 in the range from 20°C to
25°C
Resolution
pH 0.01
0.1 mV
Operating Temperature Maximum 80°C
Reaction time
≤ 1 sec for 95% of final value
When reading the pH probe output voltage, the ADC uses the
external 1.05 V reference and is configured with a gain of 1. The
full-scale input range is ±VREF/G = ±1.05 V, and the maximum
signal from the pH probe is ±490 mV at 80°C.
Because the output of the sensor is bipolar, and the AD7793
operates from a single power supply, the signal generated by the
pH probe should be biased above ground so that it is within the
acceptable common-mode range of the ADC. This bias voltage
is generated by injecting the 210 µA IOUT2 current into the
5 kΩ, 0.1% resistor as shown in Figure 2. This generates the
1.05 V common-mode bias voltage that also serves as the ADC
reference voltage.
ADC Channel 2 Configuration, RTD
The second channel of the ADC monitors the voltage generated
across an RTD being driven by the IOUT2 current output pins
of the AD7793. The 210 μA excitation current drives the series
combination of the RTD and precision resistor (5 kΩ, 0.1%).
(See Figure 1).
The temperature coefficient for pure platinum is 0.003926 Ω/Ω/°C.
The normal coefficient for industrial RTDs is 0.00385 Ω/Ω/°C
per the DIN Std. 43760-1980 and IEC 751-1983. The accuracy
of an RTD is usually stated at 0°C. The DIN 43760 standard
recognizes two classes as shown in Table 2, and ASTM E–1137
recognizes two grades as shown in Table 3.
CN-0326
Table 2. Standard RTD Accuracy for DIN-43760
Class
Tolerance
DIN 43760 Class A
±0.06% @ 0°C
DIN 43760 Class B
±0.12% @ 0°C
Table 3. Standard RTD Accuracy for ASTM E-1137
Grade
Tolerance
ASTM E-1137 Grade A ±0.05% @ 0°C
ASTM E-1137 Grade B ±0.10% @ 0°C
The RTD resistance value can be computed as
RTD Resistance = RTD₀ (1 + T α )
where:
RTD Resistance = Resistance value at T
RTD0 = Resistance value at 0°C
T = ambient temperature
α = 0.00385 Ω/Ω/°C, temperature coefficient defined by DIN
Std. 43760-1980 and IEC 751-1983
The RTD resistance varies from 0°C (1000 Ω) to 100°C (1385 Ω),
producing a voltage signal range of 210 mV to 290 mV with
210 µA excitation current.
The precision 5 kΩ resistor generates the 1.05 V used as an
external reference. With a gain of one, the analog input range is
±1.05 V (±VREF/G). This architecture gives a ratiometric
configuration. Changes in the value of the excitation current do
not affect the accuracy of the system.
Although 100 Ω Pt RTDs are popular, other resistances (200 Ω,
500 Ω, 1000 Ω, etc.) and materials (Nickel, Copper, Nickel Iron)
can be specified. This application uses a 1 kΩ DIN 43760 Class A
RTD for temperature compensation of the pH sensor. A 1000 Ω
RTD is less sensitive to wiring resistance errors than a 100 Ω RTD.
A 2-wire connection is made as shown in Figure 3. A constant
current is applied through the leads of the RTD, and the voltage
across the RTD itself is measured. The measuring device is the
AD7793 that exhibits high input impedance and low input bias
current. The sources of errors in this scheme are lead resistance,
the stability of the constant current source produced by AD7793,
and the input impedance and/or bias current in the input
amplifier, and the associated drift.
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