Datasheet
LTC2442
26
2442fa
For more information www.linear.com/LTC2442
1
5
5V
–5V
9V
4
3
2442 F19
2
6
V
OUT
V
CC
GND
C
–
LTC1983ES6-5
SHDN
C
+
C2
4.7µF
C6
4.7µF
C5
2.2µF
C3
2.2µF
C4
2.2µF
C1
4.7µF
D1
BAT54S
Input Bias Current
The 10nA typical bias current of the buffers results in less
than 1ppm (5µV) error for source resistance imbalances
of less than 500W. Matching the resistance at the inputs
cancels much of the error due to amplifier bias current.
For source resistances up to 50k, 1% resistors are ade
-
quate. Figure 20 shows proper input resistance matching
for a precision voltage divider on the CH2-3 inputs. The
resistance seen by CH2 is the parallel combination of 30k
and 10k or 7.5k. A 1%, 7.5k resistor at CH3 balances the
resistance of the divider output.
While the two input buffers will have slightly different bias
currents, the autozero process applies the bias current from
each buffer to both of the inputs for half of the conversion
time, so the offset is equal to the average of the two bias
currents multiplied by the mismatch in source resistance.
Figure 19. LTC1983 with Another Charge
Pump Stacked onto V
CC
to Give 9V
The LTC2442 breaks new ground in high impedance input
DS ADCs. The input buffer is optimized to make driving
the ADC as easy as possible, while overcoming many of
the limitations typical of integrated buffers.
Convenient +5V to –5V/+9V DC-DC
Converter
If either of the signal inputs must include ground and
V
CC
, then the amplifier will require both a positive supply
greater than the maximum input voltage and a negative
supply. Figure 19 shows how to derive both –5V and +9V
from a single 5V supply using an LTC1983, allowing the
ADC inputs to extend as much as 300mV below ground
and above V
CC
. For inputs that include ground but do
not go within 1.5V of V
CC
, then C4, C5, C6 and D1 can
be eliminated and the amplifier positive supply can be
connected to V
CC
.
applications inForMation