LTC2487_15 Datasheet, PDF (31/34 Pages) Linear Technology – 16-Bit 2-/4-Channel ADC with PGA, Easy Drive and I2C Interface

LTC2487
APPLICATIONS INFORMATION
V_CC
+
0.3V
V_CC
V_REF
2
GND
–0.3V
V_CC
V_REF
2
–V_REF
2
(a) Arbitrary
V_REF
2
V_CC
V_REF
–V_REF
2
2
GND
(b) Fully Differential
V_CC
–V_REF
2
GND
(c) Pseudo Differential Bipolar
IN– or COM Biased
Selected IN+ Ch
Selected IN– Ch or COM
GND
–0.3V
V_REF
2
–0.3V
(d) Pseudo-Differential Unipolar
IN– or COM Grounded
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Figure 37. Input Range
Easy Drive ADCs Simplify Measurement of High
Impedance Sensors
Delta-Sigma ADCs, with their high accuracy and high noise
immunity, are ideal for directly measuring many types
of sensors. Nevertheless, input sampling currents can
overwhelm high source impedances or low-bandwidth,
micropower signal conditioning circuits. The LTC2487
solves this problem by balancing the input currents, thus
simplifying or eliminating the need for signal conditioning
circuits.
A common application for a delta-sigma ADC is thermistor
measurement. Figure 38 shows two examples of thermis-
tor digitization benefiting from the Easy Drive technology.
The first circuit (applied to input channels CH0 and CH1)
uses balanced reference resistors in order to balance the
common mode input/reference voltage and balance the
differential input source resistance. If reference resistors
R1 and R4 are exactly equal, the input current is zero and
no errors result. If these resistors have a 1% tolerance,
the maximum error in measured resistance is 1.6Ω due
to a shift in common mode voltage; far less than the 1%
error of the reference resistors themselves. No amplifier
is required, making this an ideal solution in micropower
applications.
Easy Drive also enables very low power, low bandwidth
amplifiers to drive the input to the LTC2487. As shown
in Figure 38, CH2 is driven by the LT1494. The LT1494
has excellent DC specs for an amplifier with 1.5µA supply
current (the maximum offset voltage is 150µV and the
open loop gain is 100,000). Its 2kHz bandwidth makes it
unsuitable for driving conventional delta sigma ADCs. Add-
ing a 1kΩ, 0.1µF filter solves this problem by providing a
charge reservoir that supplies the LTC2487 instantaneous
current, while the 1k resistor isolates the capacitive load
from the LT1494.
Conventional delta sigma ADCs input sampling current
lead to DC errors as a result of incomplete settling in the
external RC network.
The Easy Drive technology cancels the differential input
current. By balancing the negative input (CH3) with a 1kΩ,
0.1µF network errors due to the common mode input
current are cancelled.
For more information www.linear.com/LTC2487
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