Datasheet

22
LTC1594/LTC1598
15948fb
TYPICAL APPLICATIONS N
U
Using MUXOUT/ADCIN Loop as PGA
This figure shows the LTC1598’s MUXOUT/ADCIN loop
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 LTC1594, 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 loop
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
Using the MUXOUT/ADCIN Loop of the LTC1598 to Form a PGA with Eight Gains in a Noninverting Configuration
is the summation of the resistors below the selected MUX
channel. If CH0 is selected, the loop 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
compensation for the amplifiers, help reduce the amplifi-
ers’ 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 loop gain given
by the formula above.
1594/98 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
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
5V
1 F
ADCIN
17 16 15, 19
1 F
0.1 F
5V
1 F
5V
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
LTC1598
P/ C