AD677-EB Analog Devices Inc, AD677-EB Datasheet - Page 11

• Manufacturer Part Number: AD677-EB
• Description: BOARD EVAL SAMPLING ADC AD677
• Manufacturer: Analog Devices Inc

Specifications of AD677-EB

• Rohs Status: RoHS non-compliant
• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 100k
• Data Interface: Serial
• Inputs Per Adc: 1 Single Ended
• Input Range: ±VREF
• Power (typ) @ Conditions: 450mW @ 100kSPS
• Voltage Supply Source: Analog and Digital, Dual ±
• Operating Temperature: 0°C ~ 70°C
• Utilized Ic / Part: AD677

made for selecting one with low noise. A capacitor connected
between REF IN and AGND will reduce the demands on the
reference by decreasing the magnitude of high frequency com-
ponents required to be sourced by the reference.
Figures 6 and 7 represent typical design approaches.
Figure 6 shows a voltage reference circuit featuring the 5 V out-
put AD586. The AD586 is a low cost reference which utilizes a
buried Zener architecture to provide low noise and drift. Over
the 0 C to +70 C range, the AD586M grade exhibits less than
1.0 mV output change from its initial value at +25 C. A noise
reduction capacitor, C
AD586 output, thereby optimizing the overall performance of
the AD677. It is recommended that a 10 F to 47 F high qual-
ity tantalum capacitor and a 0.1 F capacitor be tied between
the V
pedance on the reference.
Using the AD677 with 10 V input range (V
cally requires 15 V supplies to drive op amps and the voltage
reference. If 12 V is not available in the system, regulators
such as 78L12 and 79L12 can be used to provide power for the
AD677. This is also the recommended approach (for any input
range) when the ADC system is subjected to harsh environ-
ments such as where the power supplies are noisy and where
voltage spikes are present. Figure 7 shows an example of such a
system based upon the 10 V AD587 reference, which provides a
300 V LSB. Circuitry for additional protection against power
supply disturbances has been shown. A 100 F capacitor at each
REV. A
–15V
+15V
+5V
V
IN
100µF
100µF
REF
1.0µF
input of the AD677 and ground to minimize the im-
C
N
100µF
10
78L12
79L12
10µF
8
AD586
+12V
0.01µF
0.01µF
V
4
N
2
IN
10
10
10
, reduces the broadband noise of the
0.1µF
Figure 6.
Figure 7.
6
10µF
10µF
0.1µF
10µF
2
V
IN
GND
AD587
4
V
DD
V
EE
V
CC
NR
AD677
0.1µF
V
O
0.1µF
0.1µF
REF
6
8
V
REF
V
1µF
IN
V
AGND
= 10 V) typi-
AD677
REF
10µF
0.1µF
–11–
regulator prevents very large voltage spikes from entering the
regulators. Any power line noise which the regulators cannot
eliminate will be further filtered by an RC filter (10 /10 F)
having a –3 dB point at 1.6 kHz. For best results the regulators
should be within a few centimeters of the AD677.
ANALOG INPUT
As previously discussed, the analog input voltage range for the
AD677 is V
mode rejection, the V
ground. V
(AGND), and V
(AGND SENSE) which allows a remote ground sense for the
input signal.
The AD677 analog inputs (V
hibit dynamic characteristics. When a conversion cycle begins,
each analog input is connected to an internal, discharged 50 pF
capacitor which then charges to the voltage present at the corre-
sponding pin. The capacitor is disconnected when SAMPLE is
taken LOW, and the stored charge is used in the subsequent
conversion. In order to limit the demands placed on the external
source by this high initial charging current, an internal buffer
amplifier is employed between the input and this capacitance for
a few hundred nanoseconds. During this time the input pin ex-
hibits typically 20 k input resistance, 10 pF input capacitance
and 40 A bias current. Next, the input is switched directly to
the now precharged capacitor and allowed to fully settle. During
this time the input sees only a 50 pF capacitor. Once the sample
is taken, the input is internally floated so that the external input
source sees a very high input resistance and a parasitic input
capacitance of typically only 2 pF. As a result, the only domi-
nant input characteristic which must be considered is the high
current steps which occur when the internal buffers are switched
in and out.
In most cases, these characteristics require the use of an external
op amp to drive the input of the AD677. Care should be taken
with op amp selection; even with modest loading conditions,
most available op amps do not meet the low distortion require-
ments necessary to match the performance capabilities of the
AD677. Figure 8 represents a circuit, based upon the AD845,
which will provide excellent overall performance.
For applications optimized more for low distortion and low
noise, the AD845 of Figure 8 may be replaced by the AD743.
INPUT
±5V
REF
REF
1k
is referred to the local analog system ground
499
. For purposes of ground drop and common
IN
is referred to the analog ground sense pin
IN
2
3
and V
AD845
Figure 8.
+12V
–12V
IN
4
7
REF
, V
1k
inputs each have their own
REF
0.1µF
0.1µF
6
and AGND SENSE) ex-
AD677
AGND
SENSE
AGND
V
AD677
IN