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
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
µs,
and the conversion time for the
AD7572 is 5
µs
for a total of 8
µs,
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
N
]/Gain) = ([5 V/4096]/16) = 76
µV.
The
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
µV)
= 96 dB.
–15V +15V
+5V
+5V
30pF
74-
123
1/2
1/6
4
50k
10 F
+5V
V
IN
68pF
2.5MHz
68pF
AD7572
–15V +15V
+
+
10 F
10 F
+
+
10 F
5
BUSY
MSB
D12
D11
1 s
–15V +15V
–15V +15V
10 F
+
+
10 F
F
10 F
+
+
10 F
S/H
AD585
74–
LS174
D10
D9
D8
D7
1/6
6
+5V
CLOCK
125MHz
1
1/6
3
2
10k
AD526
V
IN
+5V
V
IN
S/H
AD585
B
S
47 F
D6
D5
LSB
74–
LS174
D4
D3
D2
D1
A0 A1 A2
10k
–15V +15V
10 F
+
+
AD588
2.5k
1 F
10 F
+5V
A0
10k
+5V
REF
10k
5k
10k
4
5
10k
9
10
1.25k
10k
11
1/6
10
1/4
8
1/4
6
NOTE: ALL BYPASS CAPACITORS ARE 0.1 F
74ALS86
1
3
2
1/4
A1
12
13
1/4
11
A2
74–
LS174
E1
E2
E3
1
2
1/4
3
1.25k
LM339A
Figure 42. Floating-Point A/D Converter
–12–
REV. D
AD [ ANALOG DEVICES ]
