AD526 Analog Devices, Inc., AD526 Datasheet - Page 9

• Manufacturer Part Number: AD526
• Description: Software Programmable Gain Amplifier
• Manufacturer: Analog Devices, Inc.

REV. D
The specifications on page 3, in combination with Figure 35,
give the timing requirements for loading new gain codes.
TIMING AND CONTROL
A2 A1 A0 B
X
0
0
0
0
1
X
X
0
0
0
0
1
NOTE: X = Don’t Care.
Gain Code
X
0
0
1
1
X
X
X
0
0
1
1
X
GAIN CODE
CLK OR CS
INPUTS
T
T
T
X
0
1
0
1
X
X
X
0
1
0
1
X
C
S
H
= DATA SETUP TIME
= MINIMUM CLOCK CYCLE
= DATA HOLD TIME
X
1
1
1
1
1
0
0
1
1
1
1
1
Table I. Logic Input Truth Table
CLK (CS = 0)
Figure 35. AD526 Timing
Control
T
1
0
0
0
0
0
0
1
1
1
1
1
1
C
T
VALID DATA
S
NOTE: THRESHOLD LEVEL FOR
GAIN CODE, CS, AND CLK IS 1.4V.
Condition
Previous State
T
Gain
H
16
16
1
2
4
8
1
1
1
2
4
8
Condition
Latched
Transparent
Transparent
Transparent
Transparent
Transparent
Transparent
Latched
Latched
Latched
Latched
Latched
Latched
–9–
DIGITAL FEEDTHROUGH
With either CS or CLK or both held high, the AD526 gain state
will remain constant regardless of the transitions at the A0, A1,
A2 or B inputs. However, high speed logic transitions will un-
avoidably feed through to the analog circuitry within the AD526
causing spikes to occur at the signal output.
This feedthrough effect can be completely eliminated by operat-
ing the AD526 in the transparent mode and latching the gain
code in an external bank of latches (Figure 36).
To operate the AD526 using serial inputs, the configuration
shown in Figure 36 can be used with the 74LS174 replaced by a
serial-in/parallel-out latch, such as the 54LS594.
Figure 36. Using an External Latch to Minimize Digital
Feedthrough
SIGNAL
TIMING
V
IN
A1
16
A1
1
AD526
16
LOGIC AND LATCHES
A0
15
A0
2
GAIN NETWORK
8
14
CS CLK
74LS174
3
4
13
4
2
A2
A2
12
5
1
11
B
B
6
+
–
–V
+V
10
7
S
S
+5V
9
8
0.1 F
0.1 F
OUT
FORCE
OUT
SENSE
AD526
1 F
V
OUT