AD526
THEORY OF OPERATION
TRANSPARENT MODE OF OPERATION
The AD526 is a complete software programmable gain amplifier
(SPGA) implemented monolithically with a drift-trimmed
BiFET amplifier, a laser wafer trimmed resistor network, JFET
analog switches and TTL compatible gain code latches.
A particular gain is selected by applying the appropriate gain
code (see Table I) to the control logic. The control logic turns
on the JFET switch that connects the correct tap on the gain
network to the inverting input of the amplifier; all unselected
JFET gain switches are off (open). The “on” resistance of the
gain switches causes negligible gain error since only the
amplifier’s input bias current, which is less than 150 pA, actu-
ally flows through these switches.
The AD526 is capable of storing the gain code, (latched mode),
B, A0, A1, A2, under the direction of control inputs
CLK
and
CS.
Alternatively, the AD526 can respond directly to gain code
changes if the control inputs are tied low (transparent mode).
For gains of 8 and 16, a fraction of the frequency compensation
capacitance (C1 in Figure 32) is automatically switched out of
the circuit. This increases the amplifier’s bandwidth and im-
proves its signal settling time and slew rate.
AMPLIFIER
+V
S
C1
C2
OUT
FORCE
In the transparent mode of operation, the AD526 will respond
directly to level changes at the gain code inputs (A0, A1, A2) if
B is tied high and both
CS
and
CLK
are allowed to float low.
After the gain codes are changed, the AD526’s output voltage
typically requires 5.5
µs
to settle to within 0.01% of the final
value. Figures 26 to 29 show the performance of the AD526 for
positive gain code changes.
A2
A1
A0
+V
S
+5V
0.1 F
OUT
9 FORCE
16
A1
15
14
13
12
11
B
10
A0
CS CLK
A2
LOGIC AND LATCHES
16
8
4
2
1
V
OUT
GAIN NETWORK
–
AD526
1
2
3
4
5
6
+
7
8 OUT
SENSE
0.1 F
–V
S
V
IN
V
IN
Figure 33. Transparent Mode
LATCHED MODE OF OPERATION
N1
N2
–V
S
OUT
SENSE
A0
A1
A2
B
CLK
CS
L
A
T
C
H
E
S
C
O
N
T
R
O
L
L
O
G
I
C
14k
G=8
3.4k
G=2
1k
G = 16
1.7k
RESISTOR
NETWORK
The latched mode of operation is shown in Figure 34. When
either
CS
or
CLK
go to a Logic “1,” the gain code (A0, A1, A2,
B) signals are latched into the registers and held until both
CS
and
CLK
return to “0.”
Unused
CS
or
CLK
inputs should be tied
to ground .
The
CS
and
CLK
inputs are functionally and electri-
cally equivalent.
TIMING SIGNAL
A2
A1
A0
+V
S
+5V
0.1 F
OUT
9 FORCE
G=4
DIGITAL
GND
ANALOG
GND2
1k
1.7k
ANALOG
GND1
16
A1
15
14
13
12
11
B
10
A0
CS CLK
A2
LOGIC AND LATCHES
16
8
4
2
1
V
OUT
GAIN NETWORK
–
Figure 32. Simplified Schematic of the AD526
AD526
1
2
3
4
5
6
+
7
8 OUT
SENSE
0.1 F
–V
S
V
IN
Figure 34. Latched Mode
–8–
REV. D
AD [ ANALOG DEVICES ]
