ADFS7124-8BBCPZ datasheet(75/93 Pages) AD

75 / 93 page

Data Sheet

ADFS7124-8

APPLICATIONS INFORMATION

analog.com

Rev. 0 | 75 of 93

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

filter is required to reject any interference at the modulator frequen-

cy and multiples of the modulator frequency. In addition, 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 100 mV of headroom when

buffered so a headroom resistor is then needed to bias REFIN2(−)

appropriately.

The required power mode 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 essen-

tial. In this application, the 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. The 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 vol-

tages, check all conversions and calibrations for any errors, as well

as monitor any read/write operations. In thermocouple applications,

the circuit connections are verified using the reference detect and

burnout currents. The REF_DET_ERR flag is set if the external

reference REFIN2(±) is missing. The burnout currents (available in

the configuration registers) detect an open wire. For example, if

the thermocouple is not connected and the burnout currents are

enabled on the channel, the ADC outputs a conversion that is equal

to or close to full scale. For best performance, enable the burnout

currents periodically to check the connections but disable them as

soon as the connections are verified for they add an error to the

conversions. 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/un-

dervoltage monitors are useful for detecting any excessive voltages

on AINP and AINM. The power supply voltages and reference

voltages are selectable as inputs to the ADC. Thus, the voltages

can be periodically checked to confirm whether they are within

the system specification. Also, it is possible to check that the

LDO voltages are within specification. The conversion process and

calibration process can also be checked. This ensures that any

invalid conversions or calibrations are flagged.

Finally, the CRC check, SCLK counter, and the SPI read/write

checks make the interface more robust as any read/write operation

that is not valid is detected. The CRC check highlights if any bits

are corrupted when being transmitted between the processor and

the ADC.

TEMPERATURE MEASUREMENT USING AN

RTD

To optimize a 3-wire RTD configuration, two identically matched

current sources are required. The ADFS7124-8, which contains two

well matched current sources, is ideally suited to these applications.

One possible 3-wire configuration is shown in Figure 127. In this

3-wire configuration, the lead resistances result in errors if only one

current (output at AIN0) is used, as the excitation current flows

through RL1, developing a voltage error between AIN1 and AIN2.

In the scheme outlined, the second RTD current source (available

at AIN3) compensates for the error introduced by the excitation

current flowing through RL1. The second RTD current flows through

RL2. Assuming that RL1 and RL2 are equal (the leads are normally

of the same material and of equal length) and that the excitation

currents match, the error voltage across RL2 equals the error volt-

age across RL1, and no error voltage is developed between AIN1

and AIN2. Twice the voltage is developed across RL3; however,

because this is a common-mode voltage it does not introduce

errors. The reference voltage for the ADFS7124-8 is also generated

using one of the matched current sources. It is developed using

a precision resistor and applied to the differential reference pins

of the ADC. This scheme ensures that the analog input voltage

span remains ratiometric to the reference voltage. Any errors in the

analog input voltage due to the temperature drift of the excitation

current are compensated by the variation of the reference voltage.

As an example, the PT100 measures temperature from −200°C to

+600°C. The resistance is 100 Ω typically at 0°C and 313.71 Ω at

600°C. If the 500 µA excitation currents are used, the maximum

voltage generated across the RTD when using the full temperature

range of the RTD is:

500 µA × 313.71 Ω = 156.86 mV

This is amplified to 2.51 V within the ADFS7124-8 if the gain is

programmed to 16.

The voltage generated across the reference resistor must be at

least 2.51 V. Therefore, the reference resistor value must equal at

least:

2.51 V/500 µA = 5020 Ω

Therefore, a 5.11 kΩ resistor can be used.

5.11 kΩ × Excitation Current = 5.11 kΩ × 500 µA = 2.555 V

One other consideration is the output compliance. The output com-

pliance equals AVDD − 0.37 V. If a 3.3 V analog supply is used, the

voltage at AIN0 must be less than (3.3 V − 0.37 V) = 2.93 V. From

the previous calculations, this specification is met because the

maximum voltage at AIN0 equals the voltage across the reference

resistor plus the voltage across the RTD, which equals:

2.555 V + 156.86 mV = 2.712 V

A typical procedure for reading the RTD is as follows:

1. Reset the ADC.

2. Select the power mode.

3. Set the CHANNEL_0 register analog input to AIN1/AIN2. As-

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

of 16 and select the reference source REFIN2(±). Select the

filter type and set the output data rate.

ADFS7124-8BBCPZ датащи(PDF) 75 Page - Analog Devices

ADFS7124-8BBCPZ датащи(HTML) 75 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93

датащи скачать

ссылки URL