DN379 - Easy Drive ADCs Simplify Measurement of High Impedance Sensors

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Easy Drive™ ADCs Simplify Measurement of
High Impedance Sensors – Design Note 379
Mark Thoren
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 LTC®2484
family of delta sigma converters 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 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:
⎛ VREF ⎞
⎜⎝
⎟ – VCM
V + + VIN–
2 ⎠
, where VCM = IN
1.5MΩ
2
or about 1.67μA when VREF is 5V and both inputs
are grounded.
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 amplifier is required, 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 amplifier 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
C8
1µF
C7
0.1µF
5V
R1
51.1k
IIN+ = IIN–
4
IN+
IN–
5
3
2
REF
VCC
CS
SCK
LTC2484 SDO
SDI
GND GND FO
8
6
9
7
1
10
TO IN+
4-WIRE
SPI INTERFACE
C4
0.1µF
IIN+ = 0
R3
10k-100k
TO IN–
C3
0.1µF
11
R4
51.1k
IIN
–=0
DN379 FO1
Figure 1. LTC2484 Connections
12/05/379
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 filter 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
with an ordinary delta-sigma ADC—the sampling current
from ADCs with specifications 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 filter at IN–.
5V
5V
102k
+
0.1µF
10k-100k
1k
TO IN+
LT1497
–
0.1µF
1k
TO IN–
0.1µF
DN379 F03
DN379 FO4
Figure 3. Grounded, Buffered Sensor
Figure 4. LTC2484 Demo Board
600nVRMS
DN379 FO5
Figure 5. LTC2484 Demo Software Screenshot Showing Microvolt Offset and 600nVRMS Noise
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 2453
dn379 LT/TP 1205 305K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 2005
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