7B27 AD [Analog Devices], 7B27 Datasheet - Page 2

• Manufacturer Part Number: 7B27
• Description: Isolated Thermocouple Input
• Manufacturer: AD [Analog Devices]

GENERAL DESCRIPTION
The 7B27 is a single- channel signal conditioning module that
interfaces, amplifies, and filters input voltages from a J, K, T, E,
R, S or B –type thermocouple and provides an isolated (up to
100 V rms) and protected precision output of either +1 V to +5
V or 0 V to +10V, linear with input voltage. High accuracy
internal cold junction compensation and a predictable upscale
open circuit indication provide a complete signal conditioning
solution. The 7B27 is mix-and-match and hot-swappable with
other 7B Series input modules, so can be inserted or removed
from any socket in the same backplane without disturbing
power. Model 7B27 is rated to operate with a nominal +24 VDC
supply.
The three input pins of Model 7B27 are protected up to +30
VDC. A one-pole 3 Hz filter preconditions the thermocouple
signal prior to amplification, provided by a low drift input
amplifier. Cold junction compensation (CJC) is implemented
using a thermistor temperature sensor embedded under the
input screw terminal block on a 7B Series backplane. A voltage
reference source acts to offset the thermocouple signal in
accordance with the specified lower bound on the input
temperature range. A current source provides the upscale open
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Figure 2
circuit indication. Amplitude modulation is used to implement
transformer isolation (100 V rms, continuous). Isolated front-
end circuitry power is supplied by a DC/DC converter. The
output section contains a demodulator, a two-pole low pass
filter (-3 dB @ 3 Hz), a buffer amplifier and a power oscillator.
The two-pole output filter and subsequent buffer ensures that a
low noise, low impedance (<1Ω) signal is available at the output
to drive loads down to 2 kΩ.
7B27