AD526 Analog Devices, Inc., AD526 Datasheet - Page 12

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

AD526
FLOATING-POINT CONVERSION
High resolution converters are used in systems to obtain high
accuracy, improve system resolution or increase dynamic range.
There are a number of high resolution converters available with
throughput rates of 66.6 kHz that can be purchased as a single
component solution; however in order to achieve higher through-
put rates, alternative conversion techniques must be employed.
A floating point A/D converter can improve both throughput
rate and dynamic range of a system.
In a floating point A/D converter (Figure 42), the output data is
presented as a 16-bit word, the lower 12 bits from the A/D
converter form the mantissa and the upper 4 bits from the digi-
tal signal used to set the gain form the exponent. The AD526
programmable gain amplifier in conjunction with the compara-
tor circuit scales the input signal to a range between half scale
and full scale for the maximum usable resolution.
The A/D converter diagrammed in Figure 42 consists of a pair
of AD585 sample/hold amplifiers, a flash converter, a five-range
programmable gain amplifier (the AD526) and a fast 12-bit A/D
converter (the AD7572). The floating-point A/D converter
achieves its high throughput rate of 125 kHz by overlapping the
acquisition time of the first sample/hold amplifier and the set-
tling time of the AD526 with the conversion time of the A/D
converter. The first sample/hold amplifier holds the signal for
the flash autoranger, which determines which binary quantum
10 F
+
CLOCK
125MHz
+5V
1 F
–15V
AD588
+15V
1
V
1/6
IN
2
10 F
1.25k
1.25k
2.5k
+
10k
+5V
5k
10 F
3
+
1/6
REF
4
–15V
10k
AD585
S/H
LM339A
+15V
+5V
123
+5V
74-
1/2
10k
10k
10k
10k
+
10 F
30pF
10 F
+5V
10
1
2
4
5
9
74ALS86
50k
+
Figure 42. Floating-Point A/D Converter
1/4
1/4
1/4
1 s
11
V
B
A0 A1 A2
–15V
AD526
IN
3
6
8
+5V
1/6
+15V
12
13
1
2
10
F
S
1/4
1/4
+
10 F
11
3
10 F
+
–12–
–15V
the input falls within, relative to full scale. Once the AD526 has
settled to the appropriate level, then the second sample/hold
amplifier can be put into hold which holds the amplified signal
while the AD7572 perform its conversion routine. The acquisi-
tion time for the AD585 is 3 s, and the conversion time for the
AD7572 is 5 s for a total of 8 s, or 125 kHz. This performance
relies on the fast settling characteristics of the AD526 after the
flash autoranging (comparator) circuit quantizes the input sig-
nal. A 16-bit register holds the 3-bit output from the flash autor-
anger and the 12-bit output of the AD7572.
The A/D converter in Figure 42 has a dynamic range of 96 dB.
The dynamic range of a converter is the ratio of the full-scale
input range to the LSB value. With a floating-point A/D con-
verter the smallest value LSB corresponds to the LSB of the
monolithic converter divided by the maximum gain of the PGA.
The floating point A/D converter has a full-scale range of 5 V, a
maximum gain of 16 V/V from the AD526 and a 12-bit A/D
converter; this produces:
LSB = ([FSR/2
dynamic range in dBs is based on the log of the ratio of the
full-scale input range to the LSB; dynamic range = 20 log
(5 V/76 V) = 96 dB.
10k
AD585
A0
A1
A2
S/H
+15V
NOTE: ALL BYPASS CAPACITORS ARE 0.1 F
+
10 F
47 F
2.5MHz
N
10 F
68pF
68pF
]/Gain) = ([5 V/4096]/16) = 76 V. The
+
–15V
V
AD7572
IN
BUSY
MSB
+15V
LSB
+
10 F
5
1/6
6
LS174
LS174
LS174
74–
74–
74–
+5V
REV. D
D12
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
E1
E2
E3