Circuit Note

CN-0359

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Rev. C | 7 of 10

introduce nonlinear errors to the measurements, and therefore

result in erroneous readings.

To avoid these errors, the level and frequency of the excitation

signal should be set appropriately based on the specifications of

the conductivity cell. In addition, the DC content of this waveform

should be kept to a minimum to prevent DC currents from appear-

ing across the cell electrodes.

The CN0359 is carefully designed for low DC voltage bias on the

excitation channel, with typical values being less than 200 μV.

For most commercial conductivity probes, this small residual DC

content has negligible effect. Certain probes however, depending

on their construction, can be more sensitive and might exhibit a

drift in measurement values. For these cases, capacitive coupling

should be used for the voltage measurement channel.

RTD MEASUREMENT

Conductivity measuring system accuracy is only as good as its

temperature compensation. Because common solution temperature

coefficients vary in the order of 1%/°C to 3%/°C or more, the

CN0359 implements a temperature compensation scheme. Solution

temperature coefficients are somewhat nonlinear and usually vary

with the actual conductivity as well. Therefore, calibration at the

actual measuring temperature yields the best accuracy.

The ADuCM360 contains two matched, software configurable, exci-

tation current sources. They are individually configurable to provide

a current output of 10 μA to 1 mA, and matching is better than

0.5%. The current sources allow the ADuCM360 to easily perform

2-wire, 3-wire, or 4-wire measurements for either Pt100 or Pt1000

RTDs. The software also automatically detects if the RTD is Pt100

or Pt1000 during the set up procedure.

The following discussion shows simplified schematics of how the

different RTD configurations operate. All mode switching is accom-

plished in the software, and there is no need to change the external

jumper settings.

Figure 7 shows the configuration for 4-wire RTDs.

Figure 7. Configuration for 4-Wire RTD Connection

The parasitic resistance in each of the leads to the remote RTD is

shown as RP. The excitation current passes through a precision 1.5

kΩ resistor and the RTD. The on-chip ADC measures the voltage

across the resistor.

It is important that the 1.5 kΩ resistor (REXC) and the excitation

current (IEXC) value be chosen such that the ADuCM360 maximum

input voltage at AIN7 does not exceed AVDD − 1.1 V; otherwise,

the IEXC current source does not function properly.

The RTD voltage is accurately measured using the two sense leads

that connect to AIN6 and AIN5. The input impedance is approxi-

mately 2 MΩ (unbuffered mode, PGA gain = 1), and the current

flowing through the sense lead resistance produces minimum error.

The ADC then measures the RTD voltage.

The RTD resistance is then calculated using Equation 15:

Rx=ADC6−ADC5

ADC7−ADC8

×1.5 kΩ

(15)

where:

Rx is the RTD resistance value.

ADC5 is the AIN5 input voltage in V.

ADC6 is the AIN6 input voltage in V.

ADC7 is the AIN7 input voltage in V.

ADC8 is the AIN8 input voltage in V.

The measurement is ratiometric and does not depend on an accu-

rate external reference voltage, only the tolerance of the 1.5 kΩ

resistor. In addition, the 4-wire configuration eliminates the error

associated with the lead resistances.

The ADuCM360 has a buffered or unbuffered input option. If the

internal buffer is activated, the input voltage must be greater than

100 mV. The 1 kΩ/36 Ω resistor divider provides a 115 mV bias

voltage to the RTD that allows buffered operation. In the unbuffered

mode, Terminal 4 of J3 can be grounded and connected to a

grounded shield for noise reduction.

The 3-wire connection is another popular RTD configuration that

eliminates lead resistance errors, as shown in Figure 8.

Figure 8. Configuration for 3-Wire RTD Connection

The second matched IEXC current source (AIN5/IEXC) develops

a voltage across the lead resistance in series with Terminal 3

that cancels the voltage dropped across the lead resistance in

series with Terminal 1. The measured voltage is therefore free of