Datasheet

AD8270
Rev. 0 | Page 18 of 20
DRIVING AN ADC
The AD270 high slew rate and drive capability, combined with
its dc accuracy, make it a good ADC driver. The AD8270 can
drive both single-ended and differential input ADCs. Many
converters require the output to be buffered with a small value
resistor combined with a high quality ceramic capacitor. See the
converter data sheet for more details.
Figure 51 shows the AD8270
in differential configuration, driving the
AD7688 ADC. The
AD8270 divides down the 5 V reference voltage from the
ADR435,
so that the common-mode output voltage is 2.5 V, which is
precisely where the
AD7688 needs it.
DRIVING CABLING
All cables have a certain capacitance per unit length, which varies
widely with cable type. The capacitive load from the cable may
cause peaking or instability in output response, especially when the
AD8270 is operating in a gain of 0.5.
To reduce the peaking, use a resistor between the AD8270 and the
cable. Because cable capacitance and desired output response vary
widely, this resistor is best determined empirically. A good starting
point is 20 Ω.
06979-060
AD8270
(DIFF OUT)
AD8270
(SINGLE OUT)
Figure 50. Driving Cabling
10kΩ
10kΩ
10kΩ
20kΩ
20kΩ
20kΩ
20kΩ
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
0.1µF
5V_REF
AD8270
2
3
4
5
6
7
8
9
10
11
1
15
12
–IN
+IN
–IN
+IN
16 13
+12
V
–
12
V
33Ω
2.7nF
COG
33Ω
2.7nF
COG
14
3
4
–IN
+IN
1
REF
AD7688
0.1µF
ADR435
2
4
5
GND
V
IN
V
OUT
+12V
10µF
5V_REF
NOTE:
POWER SUPPLY DECOUPLING
NOT SHOWN.
06979-037
Figure 51. Driving an ADC