ADFS7124-8BBCPZ datasheet(74/93 Pages) AD

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Data Sheet

ADFS7124-8

APPLICATIONS INFORMATION

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Rev. 0 | 74 of 93

The ADFS7124-8 offers a low cost, high resolution analog-to-digital

function. Because the analog-to-digital function is provided by a

Σ-Δ architecture, the device is more immune to noisy environments,

making it ideal for use in sensor measurement, and industrial and

process control applications.

TEMPERATURE MEASUREMENT USING A

THERMOCOUPLE

Figure 126 outlines a connection from a thermocouple to the

ADFS7124-8. In a thermocouple application, the voltage generated

by the thermocouple is measured with respect to an absolute

reference; thus, the internal reference is used for this conversion.

The cold junction measurement uses a ratiometric configuration, so

the reference is provided externally.

Because the signal from the thermocouple is small, the

ADFS7124-8 is operated with the PGA enabled to amplify the sig-

nal from the thermocouple. As the input channel is buffered, large

decoupling capacitors can be placed on the front end to eliminate

any noise pickup that may be present in the thermocouple leads.

The bias voltage generator provides a common-mode voltage so

that the voltage generated by the thermocouple is biased up to

(AVDD − AVSS)/2. For thermocouple voltages that are centered

about ground, the ADFS7124-8 can be operated with a split power

supply (±1.8 V).

The cold junction compensation is performed using a thermistor in

Figure 126. The on-chip excitation current supplies the thermistor.

In addition, the reference voltage for the cold junction measurement

is derived from a precision resistor in series with the thermistor.

This allows a ratiometric measurement so that variation of the

excitation current has no effect on the measurement (it is the ratio

of the precision reference resistance to the thermistor resistance

that is measured).

Most conversions are read from the thermocouple, with the cold

junction being read only periodically as the cold junction tempera-

ture is stable or slow moving. If a T-type thermocouple is used, it

can measure a temperature from −200°C to +400°C. The voltage

generated over this temperature range is −8.6 mV to +17.2 mV.

The ADFS7124-8 internal reference equals 2.5 V. Therefore, the

PGA is set to 128. If the thermocouple uses the AIN0/AIN1 channel

and the thermistor is connected to the AIN12/AIN13 channel, the

conversion process is as follows:

1. Reset the ADC.

2. Select the power mode. Set the CHANNEL_0 register analog

input to AIN0/AIN1. Assign Setup 0 to this channel. Configure

Setup 0 to have a gain of 128 and select the internal reference.

Select the filter type and set the output data rate.

3. Enable VBIAS on AIN0.

4. Set the CHANNEL_1 register analog input to AIN12/AIN13.

Assign Setup 1 to this channel. Configure Setup 1 to have a

gain of 1 and select the external reference REFIN2(±). Select

the filter type and set the output data rate.

5. Enable the excitation current (IOUTx) and select a suitable

value. Output this current to the AIN4 pin.

6. Enable the AIN0/AIN1 channel. Wait until RDY goes low. Read

the conversion.

7. Continue to read nine further conversions from the AIN0/AIN1

channel.

8. Disable CHANNEL_0 and enable CHANNEL_1.

9. Wait until RDY goes low. Read one conversion.

10. Repeat Step 5 to Step 8.

Using the linearization equation for the T-type thermocouple, proc-

ess the thermocouple voltage along with the thermistor voltage and

compute the actual temperature at the thermocouple head.

Figure 126. Thermocouple Application

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