Design Solutions 6 - LTC2400 Differential to Single-Ended Converter for Single 5V Supply This Converter Has High Accuracy, Very Low Offset and Offset Drift, Rail-to-Rail Input Common Mode Range and is Live at Zero

Design Solutions 6
May 1999
LTC2400 Differential to Single-Ended Converter for
Single 5V Supply
This Converter Has High Accuracy, Very Low Offset and Offset Drift, Rail-to-Rail Input
Common Mode Range and is “Live at Zero”
by Kevin R. Hoskins and Derek V. Redmayne
SPECIFICATIONS
®
VCC = VREF = LT 1019-2.5; RSOURCE = 175Ω (Balanced)
PARAMETER
Input Voltage Range
Zero Error
CIRCUIT
TOTAL
(MEASURED) LTC2400 (UNITS)
– 0.5 to 5
2
mV
1.5
µV
Input Current
See Text
Nonlinearity
±5
4
ppm
Noise (without averaging)
0.21*
1.5
µVRMS
Noise (averaged 64 readings)
0.026*
µVRMS
Resolution (with averaged readings)
17.6
Bits
Overall Accuracy (uncalibrated**)
17.6
Bits
Supply Voltage
5
5
V
Supply Current
2.6
0.2
mA
CMRR
120
dB
0 to 5
V
Common Mode Range
*Input referred noise with a gain of 101
**Does not include gain setting resistors, offset and gain error removed
OPERATION
The circuit in Figure 1 is ideal for low level differential
signals, typically 2mV/ V, in single supply applications and
features a “live at zero” operation. The circuit combines an
LTC®1043 and LTC1050 as a differential to single-ended
amplifier that has an input common mode range that
includes the power supplies. It uses the LTC1043 to
sample a differential input voltage, holds it on CS and
transfers it to a ground-referred capacitor CH, completing
the conversion to single-ended. The voltage on CH is
applied to the LTC1050’s noninverting input and amplified
by the gain set by resistors R1 and R2 (101X for the values
shown). The amplifier’s output is then converted to a
digital value by the LTC2400.
The circuit uses a simple voltage reference (the Schottky
diode and NPN transistor) to bias the single-ended signal
approximately 270mV above ground. For single supply
applications, this bias voltage and the circuit’s “live at
zero” operation allows the LTC1050 and the LTC2400 to
amplify and convert signals that include inputs below
ground.
The LTC1043 achieves its best differential to single-ended
conversion when its internal switching frequency operates at a nominal 300Hz, as set by the 0.01µF capacitor C1,
and when 1µF capacitors are used for CS and CH. CS and
CH should be a film type such as mylar or polypropylene.
Conversion accuracy is enhanced by placing a guard
shield around CS and connecting the shield to Pin 10 of the
LTC1043. This minimizes nonlinearity that results from
stray capacitance transfer errors associated with CS.
Consult the LTC1043 data sheet for more information. As
is good practice in all high precision circuits, keep all lead
lengths as short as possible to minimize stray capacitance
and noise pickup.
As stated above, the LTC1043 has the highest transfer
accuracy when using 1µF capacitors. Using any other
value will compromise the accuracy. For example, 0.1µF
will typically increase the circuit’s overall nonlinearity
tenfold.
The LTC1050’s closed-loop gain accuracy is affected by
the tolerance of the ratio of the gain-setting resistors. If
cost considerations preclude using low tolerance resistors (0.02% or better), the processor to which the LTC2400
is connected can be used to perform software correction.
Operated as a follower, the LTC1050’s gain and linearity
error is less than 0.001%.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
1
Design Solutions 6
The circuit uses 2.5V to excite the 2mV/V bridge, producing a low level output. Best performance is achieved using
bandwidth limiting as shown and the attenuator at the
LTC2400’s input to reduce the input-referred noise. The
LTC1050’s noise gain of 100 allows adequate headroom
for the expected signal magnitude. This is followed by an
attenuator that reduces the signal for an overall gain of
16.8. This gain is the typical point where the input-referred
noise is minimized.
circuitry’s connections will cause linearity perturbations
in the final conversion result. There effects can be minimized by balancing the thermocouple connections with
reversed redundant connections and by sealing the circuit
against moving air.
The circuit’s input current is dependent on the input
signal’s common mode voltage. The input current is
approximately 100nA at VIN(CM) = 5V, dropping to zero at
VIN(CM) = 0V. The values may vary from part to part. Figure
1’s input is analogous to a 2µF capacitor in parallel with a
25MΩ connected to ground. The LTC1043’s nominal
800Ω switch resistance is between the source and the 2µF
capacitance.
A source of errors is thermocouple effects that occur in
soldered connections. Their effects are most pronounced
in the circuit’s low level portion, before the LTC1050’s
output. Any temperature changes in any of the low level
5V
CH
0.1µF
5V
0.1µF
0.1µF
5V
BRIDGE—
TYPICAL
INPUT
KEEP
SHORT
4
350Ω
350Ω
2mV/V
350Ω
2
CS
1µF
(EXT)
DIFFERENTIAL
INPUT
7
CH
1µF
–
4
6
RS
5.1k
3
C1
0.1µF
5.1k
1
VCC
CS
VIN
LTC2400
SDO
FO
4
14
16
1/2 LTC1043
R2
1k
8
SCK
5
6
SERIAL
DATA OUT
7
5.1k
SERIAL
CLOCK
470Ω
350Ω
C1
0.01µF
CHIP
SELECT
470Ω
VREF
GND
R1
100k
12
13
4.3k
+
LTC1050
11
0.1µF
0.1µF
2
3
8
7
5V
5V
5V
LT1019-2.5
R3
1k
R1, R2: 0.1% OR BETTER,
10ppm/°C
'HC14 OR
EQUIVALENT
DSOL6 F01
17
1N5711
2N5210
SINGLE POINT
“STAR” CONNECTION
Figure 1. Single Supply Differential to Single-Ended Converter for
Low Level Inputs with “Live at Zero” Operation
2
Linear Technology Corporation
dsol6 LT/TP 0599 2K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
 LINEAR TECHNOLOGY CORPORATION 1999
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