LTC2440 Linear Technology, LTC2440 Datasheet - Page 23

• Manufacturer Part Number: LTC2440
• Description: 24-Bit High Speed Differential delta-sigma ADC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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 re-
quired (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
resulting 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.
Figure 17. Direct Connection to Low Impedance (<350Ω) Source
Figure 18. Input Capacitors Allow Longer Connection Between
is Possible if the Sensor is Located Close to the ADC.
REMOTE
THERMOCOUPLE
the Low Impedance Source and the ADC.
1µF
U
1µF
U
4.5V to 5.5V
IN
IN
IN
IN
+
–
+
–
4.5V to 5.5V
LTC2440
LTC2440
REF
REF
V
GND
REF
W
+
–
+
2440 F17
V
CC
2440 F18
U
1µF
1µF
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
capacitors at the inputs of the LTC2440. The 1µF capaci-
tors 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 approxi-
mately 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 resolu-
tion, 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 amplifier
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
LTC2440
sn2440, 2440fas
23