Design Solutions 1 - LTC2400 High Accuracy Differential to Single-Ended Differential to Single-Ended Converter Has Very High Uncalibrated Accuracy and Low Offset and Drift

Design Solutions 1
April 1999
LTC2400 High Accuracy Differential to Single-Ended
Converter for ± 5V Supplies
Differential to Single-Ended Converter Has Very High Uncalibrated Accuracy and
Low Offset and Drift
by Kevin R. Hoskins and Derek V. Redmayne
SPECIFICATIONS
®
VCC = VREF = LT 1236-5; VFS = 40mV;
RSOURCE = 175Ω (Balanced)
PARAMETER
Input Voltage Range
CIRCUIT
TOTAL
(MEASURED) LTC2400 (UNITS)
– 3 to 40
Zero Error
12.7
mV
1.5
µV
Input Current
See Text
Nonlinearity
±1
4
ppm
Input-Referred Noise
(without averaging)
0.3*
1.5
µVRMS
Input-Referred Noise
(averaged 64 readings)
0.05*
µVRMS
Resolution (with averaged readings)
19.6
Bits
Overall Accuracy (uncalibrated**)
18.1
Bits
±5
5
V
Supply Current
1.6
0.2
mA
CMRR
120
dB
Common Mode Range
±5
V
Supply Voltage
*Input-referred noise with a gain of 101.
**Does not include gain setting resistors.
OPERATION
The circuit in Figure 1 is ideal for low level differential
signals in applications that have a ±5V supply and need
high accuracy without calibration. 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. The voltage
on CH is applied to the LTC1050’s noninverting input and
amplified by the gain set by resistors R1 and R2 (101 for
the values shown). The amplifier’s output is then converted to a digital value by the LTC2400.
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. To
minimize the possibility of PCB leakage currents introducing an error source into CH, an optional guard circuit could
be added as shown. The common point of these two
resistors produces the potential for the guard ring. 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.
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%.
As stated above, the LTC1043 has the highest transfer
accuracy when using 1.0µF capacitors. The input current
is approximately – 100nA at VIN(CM) = – 5V, 100nA at
VIN(CM) = 5V and 0µA at VIN(CM) = 0V. For example, 0.1µF
will typically increase the circuit’s overall nonlinearity
tenfold.
Another 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
, 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 1
low level circuitry’s connections will cause linearity perturbations in the final conversion result. Their effects can
be minimized by balancing the thermocouple connections
with reversed redundant connections and by sealing the
circuit against moving air.
A subtle source of error arises from ground lead impedance differences between the LTC1043 circuit, the LTC1050
preamplifier and the LTC2400. This error can be avoided
by connecting Pin 14 of the LTC1043, the bottom end of
R2 and Pin 4 of the LTC2400 to a single-point “star”
ground.
The circuit’s input current is dependent on the input
signal’s common mode voltage. The values may vary from
5V
0.1µF
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.
The circuit schematic shows an optional resistor, RS. This
resistor can be placed in series with the LTC2400’s input
to limit current if the input goes below – 300mV. The
resistor does not degrade the converter’s performance as
long as any capacitiance, stray or otherwise, connected
between the LTC2400’s input and ground is less than
100pF. Higher capacitance will increase offset and fullscale
errors.
OPTIONAL GUARD CIRCUIT FOR CH
R4
R3
90.9Ω
9.09k
5V
BRIDGE—
TYPICAL
INPUT
VREFIN 5V
0.1µF
0.1µF
3
8
7
+
7
6
LTC1050
2
350Ω
350Ω
DIFFERENTIAL
INPUT
11
10
350Ω
+
CS
1µF
(EXT)
–
RS*
5.1k
CH
1µF
3
VIN
LTC2400
GND
0.1µF
R1
9.09k
R2
90.9Ω
12
CS
VREF
5
6
7
CHIP SELECT
SERIAL DATA OUT
SERIAL CLOCK
FO
4
8
*OPTIONAL—LIMITS INPUT CURRENT
IF THE INPUT VOLTAGE GOES BELOW
–300mV
R1, R2 = 0.02% INITIAL TOLERANCE OR BETTER
R3, R4 = 1%
14
13
SDO
SCK
4
–5V
350Ω
AGND OR
–VEXT
1
VCC
2
4
DSOL1 F01
16
C1
0.01µF
17
1/2 LTC1043
SINGLE-POINT OR “STAR” GROUND
0.1µF
–5V
Figure 1. Differential to Single-Ended Converter for Low Level Inputs,
Such as Bridges, Maintains the LTC2400’s High Accuracy
2
Linear Technology Corporation
dsol1 LT/TP 0499 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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