Datasheet

22
LTC1594L/LTC1598L
15948lfb
TYPICAL APPLICATIONS N
U
Using MUXOUT/ADCIN Loop as PGA
This figure shows the LTC1598L’s MUXOUT/ADCIN pins
and an LT1368 being used to create a single channel PGA
with eight noninverting gains. Combined with the LTC1391,
the system can expand to eight channels and eight gains
for each channel. Using the LTC1594L, the PGA is reduced
to four gains. The output of the LT1368 drives the ADCIN
and the resistor ladder. The resistors above the selected
MUX channel form the feedback for the LT1368. The gain
for this amplifier is R
S1
/R
S2
+ 1. R
S1
is the summation of
the resistors above the selected MUX channel and R
S2
is
Using the MUXOUT/ADCIN Pins of the LTC1598L to Form a PGA.
The LTC1391 MUX Allows Eight Input Channels to be Digitized
the summation of the resistors below the selected MUX
channel. If CH0 is selected, the gain is 1 since R
S1
is 0.
Table 1 shows the gain for each MUX channel. The LT1368
dual rail-to-rail op amp is designed to operate with 0.1µF
load capacitors. These capacitors provide frequency com-
pensation for the amplifiers, help reduce the amplifiers’
output impedance and improve supply rejection at high
frequencies. Because the LT1368’s I
B
is low, the R
ON
of the
selected channel will not affect the gain given by the
formula above.
1594L/98L TA06
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
20
21
22
23
24
1
2
3
64R
32R
16R
8R
4R
2R
R
R
+
–
8 COM
18 MUXOUT
= DAISY CHAIN CONFIGURATION FOR THE LTC1391 AND THE LTC1598L
GND
4, 9
10
6
5, 14
11
7
CSADC
CSMUX
CLK
D
OUT
D
IN
12
13
NC
NC
12-BIT
SAMPLING
ADC
8-CHANNEL
MUX
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
LTC1391
3V
1µF
ADCIN
17 16 15, 19
1µF
0.1µF
3V
1µF
3V
V
REF
V
CC
–
+
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
V
+
D
V
–
D
OUT
D
IN
CS
CLK
GND
1/2 LT1368
LTC1598L
µP/µC
3(5)
2(6)
1(7)
4
8