Datasheet AD693 (Analog Devices) - 9

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. Amplifier resulting in a noninverting gain of (1 + RT/100 Ω), The capacitors should be placed as close to the AD693 as where RT is the temperature dependent resistance of the RTD. possible. The value of the filter resistors should be kept low to 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 amplifier gain of +2 causing VX to be 150 mV. The Signal filter should be matched to preserve the ac common-mode 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. 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, Figure 16. Optional Input Filtering temperature) required to fill the two standard spans. Likewise,
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 Figure 17. 0-to-104°C Direct Three-Wire 100 Ω RTD lnterface, 4-20mA Output REV. A –9–