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 + R
T
/100
Ω),
The capacitors should be placed as close to the AD693 as
where R
T
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 V
X
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, V
X
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,
Table I. Precalibrated Temperature
Range Options Using a European
Standard 100
Ω
RTD and the AD693
Temperature
Range
0 to + 104°C
0 to +211°C
Pin Connections
12 to 13
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–
AD [ ANALOG DEVICES ]
