LTC2440
23
2440fe
For more information www.linear.com/LTC2440
APPLICATIONS INFORMATION
Figure 18. Input Capacitors Allow Longer Connection
Between the Low Impedance Source and the ADC.
Direct Connection to Low Impedance Sources
If the ADC can be located physically close to the sensor,
it can be directly connected to sensors or other sources
with impedances up to 350Ω with no other components
required (see Figure 17).
Longer Connections to Low Impedance Sources
If longer lead lengths are unavoidable, adding an input
capacitor close to the ADC input pins will average the
charging pulses and prevent reflections or ringing (see
Figure 18). Averaging the current pulses results in a DC
input current that should be taken into account. The re
-
sulting 110kΩ input impedance will result in a gain error
of 0.44% for a
350Ω bridge (within the full scale specs
of many bridges) and a very low 12.6ppm error for a 2Ω
thermocouple connection.
Buffering the LTC2440
Many applications will require buffering, particularly
where high impedance sources are involved or where the
device being measured is located some distance from the
LTC2440. When buffering the LTC2440 a few simple steps
should be followed.
Figure 19 shows a network suitable for coupling the inputs
of a LTC2440 to a LTC2051 chopper-stabilized op amp.
The 3µV offset and low noise of the LTC2051 make it a
good choice for buffering the LTC2440. Many other op
amps will work, with varying performance characteristics.
The LTC2051 is configured to be able to drive the 1µF ca
-
pacitors at the inputs of the LTC2440. The 1µF capacitors
should be located close to the ADC input pins.
The
measured total
unadjusted error of Figure 19 is well
within the specifications of the LTC2440 by itself. Most
autozero amplifiers will degrade the overall resolution to
some degree because of the extremely low input noise
of the LTC2440, however the LTC2051 is a good general
purpose buffer. The measured input referred noise of two
LTC2051s buffering both LTC2440 inputs is approximately
double that of the LTC2440 by itself, which reduces the
effective resolution by 1 bit for all oversample ratios. Adding
gain to the LTC2051 will increase gain and offset errors
and will not appreciably increase the overall resolution,
so it has limited benefit.
Procedure For Coupling Any Amplifier to the LTC2440
The LTC2051 is suitable for a wide range of DC and low
frequency measurement applications. If another ampli
-
fier is to be selected, a general procedure for evaluating
the suitability of an amplifier for use with the
LTC2440
is
suggested here:
1. Perform a thorough error and noise analysis on the
amplifier and gain setting components to verify that the
amplifier will perform as intended.
2. Measure the large signal response of the overall circuit.
The capacitive load may affect the maximum slew rate of
the amplifier. Verify that the slew rate is adequate for the
Figure 17. Direct Connection to Low Impedance (<350Ω) Source
is Possible if the Sensor is Located Close to the ADC.
2440 F17
REF
+
REF
–
IN
+
IN
–
LTC2440
1µF
2440 F18
1µF
4.5V to 5.5V
REMOTE
THERMOCOUPLE
V
REF
+
V
CC
GND
IN
+
IN
–
LTC2440
1µF
1µF
