Datasheet

ADA4898-1/ADA4898-2 Data Sheet
Rev. D | Page 14 of 20
THEORY OF OPERATION
The ADA4898 is a voltage feedback op amp that combines unity
gain stability with 0.9 nV/√Hz input noise. It employs a highly
linear input stage that can maintain greater than −90 dBc (at
2 V p-p) distortion out to 600 kHz while in a unity-gain
configuration. This rare combination of unity gain stability, low
input-referred noise, and extremely low distortion is the result
of Analog Devices, Inc., proprietary op amp architecture and
high voltage bipolar processing technology.
The simplified ADA4898 topology, shown in Figure 44, is a
single gain stage with a unity gain output buffer. It has over 100 dB
of open-loop gain and maintains precision specifications, such
as CMRR, PSRR, and offset, to levels that are normally associated
with topologies having two or more gain stages.
BUFFERg
m
C
C
R1 R
L
V
OUT
07037-041
Figure 44. Topology
PD (POWER-DOWN) PIN FOR THE ADA4898-1
The
PD
pin saves power by decreasing the quiescent power
dissipated in the device. It is very useful when power is an issue
and the device does not need to be turned on at all times. The
response of the device is rapid when going from power-down
mode to full power operation mode. Note that
PD
does not put
the output in a high-Z state, which means that the ADA4898
is not recommended for use as a multiplexer. Leaving the
PD
pin floating keeps the amplifier in full power operation mode.
Table 7. Power-Down Voltage Control
PD
Pin
±15 V ±10 V ±5 V
Power-Down Mode ≤−14 V ≤−9 V ≤−4 V
0.1 Hz TO 10 Hz NOISE
Figure 45 shows the 0.1 Hz to 10 Hz voltage and current noise
of the ADA4898. The peak-to-peak noise voltage is below 0.5 μV.
Figure 46 shows the circuit used to measure the low frequency
noise. It uses a band-pass filter of approximately 0.1 Hz and 10 Hz
and a high gain stage feeding into an instrumentation amplifier.
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
02 468101214161820
TIME (s)
OUTPUT VOLTAGE (µV)
07037-047
Figure 45. 0.1 Hz to 10 Hz Noise
DUT
ADA4898-1
+IN
–IN
1µF
10nF
10nF
50Ω
50Ω
+IN
–IN
AD743
–V
S
= –9V
+V
S
= +9V
1µF
15.8kΩ
13kΩ
13Ω
MOMENTARY
1kΩ
806kΩ
806kΩ
7805
7905
+V
S
= +9V
(BATTERY)
–V
S
= –9V
(BATTERY)
+V
R
= +5V
–V
R
= –5V
+V
R
= +5V
–V
R
= –5V
AD620
R
G
–IN
+IN
–V
S
R
G
+V
S
OUTPUT
REF
R = 5.36kΩ, GAIN APPROX. 10Ω
10nF
1
2
4
3
8
7
5
6
10nF
+V
S
= +9V
–V
S
= –9V
FLOATING SHIELD
COAX
TEK
TDS 754A SCOPE
IN
FARADAY CAGE
07037-048
OUT
Figure 46. Low Frequency Noise Circuit