AD74115BCPZ datasheet(39/103 Pages) AD

39 / 103 page

Data Sheet

AD74115

THEORY OF OPERATION

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Rev. 0 | 39 of 103

How to Configure a 3-Wire RTD Measurement

for Pt1000 RTD

The following is an example of how to configure a 3-wire RTD

measurement for the Pt1000 RTD:

►

Select 3-wire or 4-wire resistance measurement in the

CH_FUNC_SETUP register.

►

Set CONV1_MUX to SENSELF to SENSE_EXT1 and

CONV1_RANGE to 0 V to 12 V in the ADC_CONFIG register.

►

Set RTD_CURRENT to 500 μA and RTD_MODE_SEL to 3-wire

RTD mode in the RTD3W4W_CONFIG register.

►

Set CONV1_EN and CONV_SEQ to start continuous conver-

sions in the ADC_CONV_CTRL register.

Open-Circuit Detection

An open-circuit detect feature is available on the leads of the 3-wire

RTD. The combination of excitation current and RTD and lead

resistances generates voltages on the SENSEH and SENSE_EXT1

pins. If the voltage on either of these pins exceeds the open-circuit

detect voltage (shown in Table 7), an open-circuit signal is asserted

in the ALERT_STATUS register.

Interpreting ADC Data

In 3-wire RTD mode, configure the 16-bit, Σ-Δ ADC to measure

the voltage from SENSELF to SENSE_EXT1. When a conversion

is carried out, the ADC code reflects the ratio between RRTD and

RREF.

When using unipolar ADC ranges, use the ADC code to calculate

the RTD resistance with the following equation:

RRTD = ADC_CODE+5

65,536×ADC_GAIN

×RREF + 0.2

where:

RRTD is the calculated RTD resistance in ohms.

ADC_CODE is the code of the ADC_RESULT1 register.

RREF has a value of 2100 Ω (the combined value of the SENSEH

and RSENSE resistors).

ADC_GAIN is the gain of the ADC in the selected ADC range.

When using the 0 V to 0.625 V range (Pt100), the ADC_GAIN is 4.

When using the 0 V to 12 V range (Pt1000), the ADC_GAIN is

1/4.8.

When using bipolar ADC ranges, use the ADC code to calculate the

RTD resistance with the following equation:

RRTD = ADC_CODE ‐ 32,763

32,768 × ADC_GAIN

×RREF + 0.2

where:

RRTD is the calculated RTD resistance in ohms.

ADC_CODE is the code of the ADC_RESULT1 register.

RREF has a value of 2100 Ω (the combined value of the SENSEH

and RSENSE resistors).

ADC_GAIN is the gain of the ADC in the selected ADC range.

When using the ±104 mV range (Cu10), the ADC_GAIN is 24.

4-Wire RTD Measurements

4-wire RTD measurements are supported with the AD74115. Use

the CH_FUNC_SETUP register to configure the channel in 3-wire

or 4-wire RTD mode. Configure the RTD_MODE_SEL bit for 4-wire

RTD measurements in the RTD3W4W_CONFIG register.

Figure 37 shows a simplified configuration of 4-wire RTD method.

An excitation current, I1 is sourced to a single lead of the RTD via

SENSEH. The fourth lead is connected to ground.

There is no current flow in second and third leads of the RTD that

are connected to SENSE_EXT2 and SENSE_EXT1, respectively;

therefore, these pins are used to sense the voltage directly across

the RTD.

The full-scale range of the ADC is determined by the voltage across

RREF, guaranteeing a fully ratiometric measurement.

The excitation current applied to the RTD terminal can be pro-

gramed to one of four values between 250 µA to 1 mA using

the RTD_CURRENT bits in the RTD3W4W_CONFIG register. See

Table 7 for the full list of excitation currents. Select the excitation

current according to the RTD in use. Take care that the voltage

generated on the SENSEHF pin (I1 × (RREF + RRTD)) is less than

VAVCC. The SENSEHF pin voltage provides the positive reference

to the ADC and must not exceed the value of VAVCC.

The measurement range can be configured in the ADC_CONFIG

suit the RTD in use.

AD74115BCPZ Datasheet(PDF) 39 Page - Analog Devices

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