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SHORT CYCLING THE 8-PIN ADS78xx FAMILY
By Bonnie C. Baker
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The most recent 8-pin, 12-bit A/D converters that Burr-
Brown has introduced to the market are advertised as high
speed, micro power, low priced sampling SAR devices.
During product development and the initial introduction
period, the engineering effort was placed on optimizing the
combination of 12 bits with high speed, low power and low
cost. Burr-Brown Applications has revisited these converters
to find that the claims of high speed and micro power at
12 bits are only part of the story. If power savings is a
priority and fewer bits are tolerable in the application, these
parts can be operated at much higher speeds and with
considerably lower power dissipation. The products that are
discussed in this application note are the ADS7816, ADS7817
and ADS7822 (Figure 1).
The data sheet key specifications for these products are
shown in Table I. The maximum through-put rate stated in
Table I assumes that the devices operate at that speed with
less error than stated in the maximum linearity error speci-
fications. This is, of course, in a 12-bit system. The differen-
tiating specifications between these three products are speed,
power, and supply voltage. As shown in Table I, the power
dissipation of the ADS7816 at a 200kHz continuous sam-
pling rate is 1.9mW. If the application allows for a conser-
vative 16 clock cycles to complete the conversion, the clock
rate for the ADS7816 and ADS7817 would be 3.2MHz and
for the ADS7822, 1.2MHz.
3
4
SAR
Control
V
REF
D
OUT
+In
CDAC
鈥揑n
S/H Amp
Comparator
Serial
Interface
DCLOCK
CS/SHDN
Figure 1. The Basic Topology of the ADS7816, ADS7817, and ADS7822 is Shown in this Figure. All three devices are
12-bit, 8-pin, SAR converters with serial outputs.
ADS7817C
(V
REF
= 2.5V)
Maximum Through-put Rate, 1/t
CYC
Maximum system clock rate assuming 16
clocks for one conversion plus data transfer
Typical Power Dissipation at Specified
Through-put Rate
Maximum Integral Linearity Error
Maximum Differential Linearity Error
Power Supply for above Specifications
Input Structure
200kHz
3.2MHz
2.3mW
鹵1LSB
鹵1LSB
5.0V
Fully Differential
ADS7816C
(V
REF
= 5V)
200kHz
3.2MHz
1.9mW
鹵1LSB
鹵0.75LSB
5.0V
Single-Ended with
Sampled Ground Sense
ADS7822C
(V
REF
= 3V)
75kHz
1.2MHz
0.54mW
鹵0.75LSB
鹵0.75LSB
2.7V
Single-Ended with
Sampled Ground Sense
TABLE I. Published Specifications of Three of the 12-Bit, Micro-power, Sampling A/D Converters.
漏
1997 Burr-Brown Corporation
AB-117
1
Printed in U.S.A. June, 1997
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