ADA4255ACPZ-R7 datasheet(42/64 Pages) AD

42 / 64 page

Data Sheet

ADA4255

APPLICATIONS INFORMATION

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Rev. 0 | 42 of 64

3-WIRE RTD WITH CURRENT EXCITATION

3-wire RTDs are commonly used for precision temperature meas-

urement. Figure 108 shows how the ADA4255 can be used to

accurately measure temperature using a 3-wire RTD sensor. In

this implementation, the current source of the ADA4255, IOUT, is

used to drive the RTD. RL1, RL2, and RL3 represent the parasitic

lead resistances of the RTD. Through a sequence of three voltage

measurements and by assuming all RLx resistors are equal, a

temperature measurement can be made that is insensitive to the

parasitic resistances of RL1, RL2 and RL3. Refer to Figure 108 to

aid in the following measurement description.

The excitation current flows through RL1, RTD, RL3, and RREF.

RREF serves as a current sense resistor used to measure the true

value of IOUT. Because of this, the tolerance and drift of RREF

are important in achieving system accuracy specifications. The

combined voltage on RTD and RL1 can be measured between

+IN1 and −IN1. Note that the portion of this measured voltage

that is on RL1 is an error term, and it matches the voltage on

RL3 because RL1 matches RL3, and the same current flows in

both. Next, measure the voltage between −IN2 and +IN2 with the

known value of RREF to calculate the true value of IOUT. A final

measurement of the voltage between −IN1 and +IN2 results in

the combined voltage on RL3 and RREF. From these three voltage

measurements, the voltage across RTD and the current conducted

in RTD are determined, and the RTD value is calculated and used

to determine temperature.

The gain of the ADA4255 must be optimized for each of these

three measurements to maximize resolution. To achieve some of

the switch combinations, the MUX_PROT_DIS bit

(Register ANALOG_ERR_DIS) must also be set.

The ADA4255 internal chopping circuitry can be synchronized to

the companion ADC to help keep the residual chopping noise at

its frequency and to prevent the noise from folding back into a

frequency band of interest. To use the synchronization functionality,

configure GPIO4 to be an input by setting its corresponding bit in

the GPIO_DIR register. Set the ADA4255 to accept an external

clock by setting the EXT_CLK_IN bit in the SF_CFG register.

Adjust the clock divider such that the resulting clock is equal to

1 MHz. The divider can be adjusted in SYNC_CFG register. The

SYNC_CFG register also controls the syncing edge polarity. It is

recommended that two reads from the M_CLK_CNT register are

performed to ensure that the master clock counter is incrementing,

indicating that the ADA4255 is getting an external clock.

The ADA4255 on-chip diagnostics allow the user to check the

circuit connections. In RTD applications, the circuit connections are

verified using the wire break detection capabilities of the ADA4255.

The WB_DETECT register flag is set if one of the RTD wires

is missing. Finally, the CRC check, SCLK counter, and SPI read

and/or write check make the interface more robust because any

read and/or write operations that are not valid are detected. The

CRC check highlights if any bits are corrupted when transmitted

between the processor and the ADA4255.

Figure 108. 3-Wire RTD Application

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