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LTC2489

2489f

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 LTC2489

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 27 shows two examples of thermistor

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 LTC2489. As shown

in Figure 27, 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.

Adding a 1kΩ, 0.1μF ﬁlter solves this problem by providing

a charge reservoir that supplies the LTC2489 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.

APPLICATIONS INFORMATION

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