ADC7802BN Burr-Brown Corporation, ADC7802BN Datasheet - Page 10

• Manufacturer Part Number: ADC7802BN
• Description: Autocalibrating, 4-channel, 12-bit Analog-to-digital Converter
• Manufacturer: Burr-Brown Corporation

INSTALLATION
INPUT BANDWIDTH
From the typical performance curves, it is clear that ADC7802
can accurately digitize signals up to 500Hz, but distortion
will increase beyond this point. Input signals slewing faster
than 8mV/ s can degrade accuracy. This is a result of the
high-precision auto-zeroing circuit used during the acquisi-
tion phase. For applications requiring higher signal band-
width, any good external sample/hold, like the SHC5320,
can be used.
INPUT IMPEDANCE
ADC7802 has a very high input impedance (input bias
current over temperature is 100nA max), and a low 50pF
input capacitance. To ensure a conversion accurate to 12 bits,
the analog source must be able to charge the 50pF and settle
within the first five clock cycles after a conversion is initi-
ated. During this time, the input is also very sensitive to noise
at the analog input, since it could be injected into the
capacitor array.
FIGURE 7. Measuring Active LOW to/from Hi-Z State.
Enable
Enable
Output
Output
Gnd
Gnd
V
V
®
V
V
V
V
OL
OL
ADC7802
D
D
D
D
ADC7802
Output
90%
(b) From LOW to Hi-Z, C = 10pF
(c) From Hi-Z to LOW, C = 50pF
50%
10%
t
FALL
(a) Load Circuit
t
RISE
t
10%
15
5V
t
50%
14
3k
90%
t
C
13
L
10%
Point
L
Test
L
0.8V
10
FIGURE 8. Measuring Active HIGH to/from Hi-Z State.
In many applications, a simple passive low-pass filter as
shown in Figure 9a can be used to improve signal quality. In
this case, the source impedance needs to be less than 5k to
keep the induced offset errors below 1/2LSB, and to meet the
acquisition time of five clock cycles. The values in Figure 9a
meet these requirements, and will maintain the full power
bandwidth of the system. For higher source impedances, a
buffer like the one in Figure 9b should be used.
FIGURE 9. Input Signal Conditioning.
Output
Enable
Output
Enable
Analog
Analog
Input
Input
Gnd
Gnd
Gnd
Gnd
V
V
V
V
OH
OH
D
D
ADC7802
Output
R
(b) From HIGH to Hi-Z, C = 10pF
(a) Passive Low Pass Filter
C
(b) Active Low Pass Filter
100
90%
10%
(c) From Hi-Z to HIGH, C = 50pF
50%
3k
V
(a) Load Circuit
REF
t
OPA27
FALL
t
RISE
– (Normally 0V)
50%
10%
V
t
15
t
90%
t
22nF
14
13
REF
– (Normally 0V)
C
L
L
Point
90%
Test
2.4V
L
To ADC7802
To ADC7802