LTC2446 LINER [Linear Technology], LTC2446 Datasheet - Page 25

• Manufacturer Part Number: LTC2446
• Description: 24-Bit High Speed 8-Channel ?? ADCs with Selectable Multiple Reference Inputs
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Automatic Offset Calibration of External
Buffers/Amplifiers
The LTC2447 enables an external amplifier to be inserted
between the multiplexer output and the ADC input. This
enables one external buffer/amplifier circuit to be shared
between all nine analog inputs (eight single-ended or four
differential). The LTC2447 performs an internal offset
calibration every conversion cycle in order to remove the
offset and drift of the ADC. This calibration is performed
through a combination of front end switching and digital
processing. Since the external amplifier is placed between
the multiplexer and the ADC, it is inside the correction
loop. This results in automatic offset correction and offset
drift removal of the external amplifier.
The LT1368 is an excellent amplifier for this function.
It has rail-to-rail inputs and outputs, and it operates on a
single 5V supply. Its open-loop gain is 1M and its input
bias current is 10nA. It also requires at least a 0.1µF load
capacitor for compensation. It is this feature that sets
it apart from other amplifiers—the load capacitor
Figure 17. External Buffers Provide High Impedance Inputs and Amplifier Offsets are Cancelled
U
10
9
U
DIFFERENTIAL
REFERENCE
CONDITIONING CIRCUITS
INPUTS
OFFSETS AND 1/f NOISE
CH0-CH6/
FIVE
OF EXTERNAL SIGNAL
ARE AUTOMATICALLY
COM
W
CANCELLED
MUX
MUX
U
(EXTERNAL AMPLIFIERS)
2
3
6
5
–
+
–
+
LTC2447
1/2 LT1368
1/2 LT1368
5V
0V
8
4
attenuates sampling glitches from the LTC2447 ADCIN
terminal, allowing it to achieve full performance of the
ADC with high impedance at the multiplexer inputs.
Another benefit of the LT1368 is that it can be powered
from supplies equal to or greater than that of the ADC.
This can allow the inputs to span the entire absolute
maximum of GND – 0.3V to V
supply of 7.5V to 10V and a negative supply of –2.5 to
–5V gives the amplifier plenty of headroom over the
LTC2447 input range.
Interfacing Sensors to the LTC2447
Figure 18 shows a few of the ways that the multiple
reference inputs of the LTC2447 greatly simplify sensor
interfacing. Each of the four references is fully differential
and has a differential range of 100mV to 5V. This opens up
many possibilities for sensing voltages and currents,
eliminating much of the analog signal conditioning cir-
cuitry required for interfacing to conventional ADCs.
0.1µF*
1
7
0.1µF*
24467 F17
*LT1368 REQUIRES 0.1µF
OUTPUT COMPENSATION
CAPACITOR
∆Σ ADC
SPEED
REF
REF
HIGH
+
–
LTC2446/LTC2447
SDO
SCK
SDI
CS
CC
+ 0.3V. Using a positive
25
24467fa