AD670
APPLYING THE AD670
The AD670 has been designed for ease of use, system compat-
ibility, and minimization of external components. Transducer
interfaces generally require signal conditioning and preampli-
fication before the signal can be converted. The AD670 will
reduce and even eliminate this excess circuitry in many cases.
To illustrate the flexibility and superior solution that the AD670
can bring to a transducer interface problem, the following dis-
cussions are offered.
Temperature Measurements
˜
Temperature transducers are one of the most common sources
of analog signals in data acquisition systems. These sensors re-
quire circuitry for excitation and preamplification/buffering. The
instrumentation amplifier input of the AD670 eliminates the
need for this signal conditioning. The output signals from tem-
perature transducers are generally sufficiency slow that a
sample/hold amplifier is not required. Figure 12 shows tile
AD590 IC temperature transducer interfaced to the AD670.
The AD580 voltage reference is used to offset the input for 0°C
calibration. The current output of the AD590 is converted into
a voltage by R1. The high impedance unbuffered voltage is ap-
plied directly to the AD670 configured in the –128 mV to
127 mV bipolar range. The digital output will have a resolution
of 1°C.
Figure 13. Low Cost RTD Interface
Differential temperature measurements can be made using an
AD590 connected to each of the inputs as shown in Figure 14.
This configuration will allow the user to measure the relative
temperature difference between two points with a 1°C resolu-
tion. Although the internal 1k and 9k resistors on the inputs
have ±20% tolerance, trimming the AD590 is unnecessary as
most differential temperature applications are concerned with
the relative differences between the two. However, the user may
see up to a 20% scale factor error in the differential temperature
to digital output transfer curve.
This scale factor error can be eliminated through a software cor-
rection. Offset corrections can be made by adjusting for any dif-
ference that results when both sensors are held at the same
temperature. A span adjustment can then be made by immers-
ing one AD590 in an ice bath and one in boiling water and
eliminating any deviation from 100°C. For a low cost version of
this setup, the plastic AD592 can be substituted for the AD590.
Figure 12. AD670 Temperature Transducer lnterface
Platinum RTDs are also a popular, temperature transducer.
Typical RTDs have a resistance of 100 Ω at 0°C and change re-
sistance 0.4 Ω per °C. If a consent excitation current is caused
to flow in the RTD, the change in voltage drop will be a mea-
sure of the change in temperature. Figure 13 shows such a
method and the required connections to the AD670. The
AD580 2.5 V reference provides the accurate voltage for the ex-
citation current and range offsetting for the RTD. The op amp
is configured to force a constant 2.5 mA current through the
RTD. The differential inputs of the AD670 measure the differ-
ence between a fixed offset voltage and the temperature depen-
dent output of the op amp which varies with the resistance of
the RTD. The RTD change of approximately 0.4 Ω/°C results
in a 1 mV/°C voltage change. With the AD670 in the 1 mV/LSB
range, temperatures from 0°C to 255°C can be measured.
Figure 14. Differential Temperature Measurement
Using the AD590
REV. A
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ADI [ ADI ]
