LINEAR TECHNOLOGY LTC2484 Technical data

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Easy Drive™ ADCs Simplify Measurement of
High Impedance Sensors – Design Note 379

Mark Thoren

Delta-si gma ADCs, with their high accuracy a nd 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 LTC

2484
family of delta sigma converters solves this problem by
balancing the input currents, thus simplif ying or eliminat­ing the need for signal conditioning circuits.

A common applicat ion for a delta-sigma ADC is thermis tor
measurement. Figure 1 shows the LTC2484 connections
for direct measurement of thermistors up to 100kΩ. Data
I/O is through a standard SPI interface and the sampling
current in each input is approximately:

V

⎛

⎞

REF

–

V

⎜

⎝

2

15 2

or about 1.67μA when V

CM

⎟

⎠

.

M

Ω

,

where V

CM

REF

+

–

+

VV

IN IN

=

is 5V and both inputs

are grounded.

5V

6
9
7
1
10

C7

0.1µF

4-WIRE
SPI INTERFACE

C8
1µF

IN

IN

+

–

32

REF

V

LTC2484

GND

GND

CC

CS

SCK

SDO

SDI

F

O

118

+

–

= I

I

IN

IN

4

5

Figure 2 shows how to balance the thermistor such
that the ADC input current is minimized. If reference
resistors R1 and R4 are exactly equal, the input current
is zero and no errors result. If the reference resistors
have a 1% tolerance, the maximum error in the measured
resistance is 1.6Ω due to the slight shift in common mode
voltage; far less than the 1% error of the reference resis­tors themselves. No amplifi er i s r equir ed, making this an
ideal solution in micropower applications.

It may be necessary to ground one side of the sensor to
reduce noise pickup or simplify wiring if the sensor is
remote. The varying common mode voltage produces a

3.5kΩ full-scale error in the measured resistance if this
circuit is used without buffering.

Figure 3 shows how to interface a very low power, low

®

bandwidth op amp to the LTC2484. The LT

1494 has

excellent DC specs for an amplifi er with 1.5µA supply

, LTC and LT are registered trademarks and Easy Drive is a trademark of Linear
Technology Corporation. All other trademarks are the property of their respective
owners.

5V

R1

51.1k

+

TO IN

+

I

= 0

IN

–

TO IN

–

= 0

I

IN

0.1µF

0.1µF

C4

C3

R3
10k-100k

R4

51.1k

12/05/379

Figure 1. LTC2484 Connections

DN379 FO1

DN379 FO2

Figure 2. Centered Sensor

current—the maximum offset voltage is 150µV and
the open loop gain is 100,000—but its 2kHz bandwidth
makes it unsuitable for driving conventional delta-sigma
ADCs. Adding a 1kΩ, 0.1µF fi lter solves this problem by
providing a charge reservoir that supplies the LTC2484’s
instantaneous sampling current, while the 1kΩ resistor
isolates the capacitive load from the LT1494. Don’t try this

5V

with an ordinary delta-sigma ADC—the sampling current
from ADCs with specifi cations similar to the LTC2484
family would result in a 1.4mV offset and a 0.69mV
full-scale error in the circuit shown in Figure 3. The
LTC2484’s balanced input current allows these errors to

–

be easily cancelled by placing an identical fi lter at IN

.

102k

10k-100k

0.1µF

+

–

5V

LT1497

1k

0.1µF

1k

0.1µF

DN379 F03

TO IN

TO IN

+

–

DN379 FO4

Figure 4. LTC2484 Demo BoardFigure 3. Grounded, Buffered Sensor

Figure 5. LTC2484 Demo Software Screenshot Showing Microvolt Offset and 600nV

Data Sheet Download

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Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

600nV

RMS

DN379 FO5

Noise

RMS

For applications help,

call [phone], Ext. 2453

dn379 LT/TP 1205 305K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2005