ADFS7124-8BBCPZ datasheet(77/93 Pages) AD

77 / 93 page

Data Sheet

ADFS7124-8

APPLICATIONS INFORMATION

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Rev. 0 | 77 of 93

FLOWMETER

Figure 128 shows the ADFS7124-8 being used in a flowmeter

application that consists of two pressure transducers, with the

rate of flow being equal to the pressure difference. The pressure

transducers are arranged in a bridge network and give a differential

output voltage between its OUT+ and OUT− terminals. With rated

full-scale pressure (in this case, 300 mmHg) on the transducer, the

differential output voltage is 3 mV/V of the input voltage (that is, the

voltage between the IN+ and IN− terminals).

Assuming a 3 V excitation voltage, the full-scale output range from

the transducer is 9 mV. The excitation voltage for the bridge can

directly provide the reference for the ADC, as the reference input

range includes the supply voltage.

A second advantage of using the ADFS7124-8 in transducer-based

applications is that the low-side power switch can be fully utilized

in low power applications. The low-side power switch is connected

in series with the cold side of the bridges. In normal operation,

the switch is closed and measurements are taken. In applications

where power is of concern, the ADFS7124-8 can be placed in

the standby mode, thus significantly reducing the power consumed

in the application. In addition, the low-side power switch can be

opened while in the standby mode, thus avoiding unnecessary

power consumption by the front-end transducers. When the device

is taken out of the standby mode, and the low-side power switch

is closed, ensure that the front-end circuitry is fully settled before

attempting a read from the ADFS7124-8. The power switch can

be closed before taking the device out of standby, if needed. This

allows time for the sensor to power up and settle before the ADC

powers up and begins sampling the analog input.

In the diagram, temperature compensation is performed using a

thermistor. The on-chip excitation current supplies the thermistor. In

addition, the reference voltage for the temperature 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).

If the sensor sensitivity is 3 mV/V and the excitation voltage is 3

V, the maximum output from the sensor is 9 mV. The ADFS7124-8

PGA can be set to 128 to amplify the sensor signal.

The ADFS7124-8 PGA amplifies the signal to:

9 mV × 128 = 1.152 V

This value does not exceed the reference voltage (3 V).

A typical procedure for reading the sensors is as follows:

1. Reset the ADC.

2. Select the power mode.

3. Set the CHANNEL_0 register analog input to AIN0/AIN1. As-

sign Setup 0 to this channel. Configure Setup 0 to have a gain

of 128 and select the reference source to REFIN1(±). Select the

filter type and set the output data rate.

4. Set the CHANNEL_1 register analog input to AIN2/AIN3. As-

sign Setup 0 to this channel (both channels use the same

setup).

5. Set the CHANNEL_2 register analog input to AIN4/AIN5. As-

sign Setup 1 to this channel. Configure Setup 1 to have a gain

of 1 and select the reference source REFIN2(±). Select the filter

type and set the output data rate.

6. Program the excitation current and output the current on the

AIN4 pin.

7. Enable both CHANNEL_0 and CHANNEL_1. Enable the DA-

TA_STATUS bit to identify the channel from which the conver-

sion originated. The ADC automatically sequences through

these channels.

8. Wait until RDY goes low. Read the conversion value.

9. Repeat Step 8 until the temperature is to be read (every 10

conversions of the pressure sensor readings, for example).

10. Disable CHANNEL_0 and CHANNEL_1. Enable CHANNEL_2.

11. Wait until RDY goes low. Read the conversion.

12. Repeat Step 6 to Step 10.

In the processor, the conversion information is converted to pres-

sure and the flow rate can be calculated. The processor typically

contains a lookup table for each pressure sensor so its variation

with temperature can be compensated.

The external antialias filter is omitted for clarity. However, such a fil-

ter is required to reject any interference at the modulator frequency

and multiples of the modulator frequency. Also, some filtering may

be needed for EMI purposes. Both the analog inputs and reference

inputs can be buffered, which allows the user to connect any RC

combination to the reference or analog input pins. Note that the

reference inputs require 100mV of headroom when buffered so

headroom resistors are then required.

The power mode to use depends on the performance required from

the system along with the current consumption allowance for the

system. In a field transmitter, low current consumption is essential.

In this application, low power mode or mid power mode is most

suitable. In process control applications, power consumption is not

a priority. Thus, full power mode may be selected. Full power mode

offers higher throughput and lower noise.

The ADFS7124-8 on-chip diagnostics allow the user to check the

circuit connections, monitor power supply, reference, and LDO

voltages, check all conversions and calibrations for any errors, as

well as monitor any read/write operations. The REF_DET_ERR flag

is set if the external reference REFIN2(±) or REFIN1(±) is missing.

The decoupling capacitors on the LDOs can also be checked. The

ADC indicates if the capacitor is not present.

As part of the conversion process, the analog input overvoltage/

undervoltage monitors are useful to detect any excessive voltages

on AINP and AINM. The power supply voltages and reference

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