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AD693AE

Loop-Powered 4–20 mA Sensor Transmitter

Analog Devices 

AD693

Figure 15. Local Powered Operation with 0–20 mA Output

OPTIONAL INPUT FILTERING

INTERFACING PLATINUM RTDS

Input filtering is recommended for all applications of the

The AD693 has been specially configured to accept inputs from

AD693 due to its low input signal range. An RC filter network 100 Ω Platinum RTDs (Resistance Temperature Detectors).

at each input of the signal amplifier is sufficient, as shown in

Referring to Figure 17, the RTD and the temperature stable

Figure 16. In the case of a resistive signal source it may be

100 Ω resistor form a feedback network around the Auxiliary

necessary only to add the capacitors, as shown in Figure 18.

The capacitors should be placed as close to the AD693 as

possible. The value of the filter resistors should be kept low to

Amplifier resulting in a noninverting gain of (1 + RT/100 Ω),

where RT is the temperature dependent resistance of the RTD.

The noninverting input of the Auxiliary Amplifier (Pin 2) is

minimize errors due to input bias current. Choose the 3 dB

then driven by the 75 mV signal from the Voltage Divider (Pin

point of the filter high enough so as not to compromise the

4). When the RTD is at 0, its 100 Ω resistance results in an

bandwidth of the desired signal. The RC time constant of the

filter should be matched to preserve the ac common-mode

amplifier gain of +2 causing VX to be 150 mV. The Signal

Amplifier compares this voltage to the 150 mV output (Pin 3) so

rejection.

that zero differential signal results. As the temperature (and

therefore, the resistance) of the RTD increases, VX will likewise

increase according to the gain relationship. The difference

between this voltage and the zero degree value of 150 mV drives

the Signal Amp to modulate the loop current. The AD693 is

precalibrated such that the full 4-20mA output span corresponds

to a 0 to 104°C range in the RTD. (This assumes the European

Standard of α = 0.00385.) A total of 6 precalibrated ranges for

three-wire (or two-wire) RTDs are available using only the pin

strapping options as shown in Table I.

Figure 16. Optional Input Filtering

A variety of other temperature ranges can be realized by using

different application voltages. For example, loading the Voltage

Divider with a 1.5 kΩ resistor from Pin 3 to Pin 6 (common)

will approximately halve the original application voltages and

allow for a doubling of the range of resistance (and therefore,

temperature) required to fill the two standard spans. Likewise,

Figure 17. 0-to-104°C Direct Three-Wire 100 Ω RTD lnterface, 4-20mA Output

REV. A

–9–

Table I. Precalibrated Temperature

Range Options Using a European

Standard 100 Ω RTD and the AD693

Temperature

Range

Pin Connections

0 to + 104°C

12 to 13

0 to +211°C

12 to 13, and

15 to 16

+25°C to +130°C 12 to 14

+51°C to +266°C 12 to 14, and

15 to 16

–50°C to +51°C 12 to 11

–100°C to +104°C 12 to 11 and

15 to 16

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