AD8671 AD8672 AD8674 Analog Devices, AD8671 AD8672 AD8674 Datasheet - Page 11

• Manufacturer Part Number: AD8671 AD8672 AD8674
• Description: Precision, Very Low Noise, Low Input Bias Current Operational Amplifiers
• Manufacturer: Analog Devices

APPLICATIONS
FOLLOWER APPLICATIONS
When large transient pulses (>1 V) are applied at the positive
terminal of amplifiers (such as the OP27, LT1007, OPA227, and
AD8671) with back-to-back diodes at the input stage, the use of
a resistor in the feedback loop is recommended to avoid having
the amplifier load the signal generator. The feedback resistor, R
should be at least 500 Ω. However, if large values must be used
for R
R
capacitance and R
Figure 32 shows the uncompensated output response with a
10 kΩ resistor in the feedback and the compensated response
with C
OUTPUT PHASE REVERSAL
Phase reversal is a change of polarity in the amplifier transfer
function, which occurs when the input voltage exceeds the
supply voltage. The AD8671/AD8672/AD8674 do not exhibit
phase reversal even when the input voltage is 1 V beyond the
supplies.
F
to compensate for the pole introduced by the input
F
, a small capacitor, C
F
= 15 pF.
Figure 32. Transient Output Response
F
.
F
, should be inserted in parallel with
REF1 +OVER
23.23%
CH2 +OVER
7.885%
03726-0-022
Rev. A | Page 11 of 16
F
,
TOTAL NOISE VS. SOURCE RESISTANCE
The low input voltage noise of the AD8671 makes it a great
choice for applications with low source resistance. However,
because the AD8671 has low input current noise, it can also be
used in circuits with substantial source resistance.
Figure 34 shows the voltage noise, current noise, thermal noise,
and total rms noise of the AD8671 as a function of the source
resistance.
For R
For 475 Ω < R
For R
S
S
1000
< 475 Ω, the input voltage noise, e
> 412 kΩ, the input current noise dominates.
100
10
1
10
S
< 412 kΩ, thermal noise dominates.
Figure 34. Noise vs. Source Resistance
Figure 33. Output Phase Reversal
100
e
n_t
SOURCE RESISTANCE (Ω)
A
AD8671/AD8672/AD8674
TIME (10µs/DIV)
1k
V
IN
(4kR
S
T)
10k
1/2
B
n
V
, dominates.
OUT
V
100k
SY
e
= ±15V
n
03726-0-020
03726-0-028
i
C
n
1M