AD797
20pF TO 120pF
100Ω
DRIVING CAPACITIVE LOADS
R
1
The capacitive load driving capabilities of the AD797 are
displayed in Figure 41. At gains greater than 10, usually no
special precautions are necessary. If more drive is desirable,
however, the circuit shown in Figure 42 should be used. For
example, this circuit allows a 5000 pF load to be driven cleanly
at a noise gain ≥2.
+V
S
*
I
IN
2
3
7
6
*
AD797
600Ω
4
100nF
C
R
S
S
–V
S
10nF
1nF
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 39. I-to-V Converter Connection
THE INVERTING CONFIGURATION
The inverting configuration (see Figure 40) presents a low input
impedance, R1, to the source. For this reason, the goals of both
low noise and input buffering are at odds with one another.
Nonetheless, the excellent dynamics of the AD797 makes
it the preferred choice in many inverting applications, and
with careful selection of feedback resistors, the noise penalties
are minimal. Some examples are presented in Table 5 and
Figure 40.
100pF
10pF
1pF
1
10
100
1k
CLOSED-LOOP GAIN
Figure 41. Capacitive Load Drive Capability vs. Closed-Loop Gain
C
L
20pF
R
2
1kΩ
+V
7
S
200pF
100Ω
*
R1
+V
S
2
3
*
6
*
AD797
1kΩ
2
3
7
R
L
4
33Ω
6
*
AD797
R
S
C1
4
–V
S
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
–V
S
Figure 40. Inverting Amplifier Connection
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Table 5. Values for Inverting Circuit
Figure 42. Recommended Circuit for Driving a High Capacitance Load
Noise
SETTLING TIME
Gain
−ꢀ
−ꢀ
R1
R2
CL
(Excluding RS)
3.0 nV/√Hz
ꢀ.8 nV/√Hz
ꢀ.8 nV/√Hz
The AD797 is unique among ultralow noise amplifiers in that it
settles to 16 bits (<150 μV) in less than 800 ns. Measuring this
performance presents a challenge. A special test circuit (see
Figure 43) was developed for this purpose. The input signal was
obtained from a resonant reed switch pulse generator, available
from Tektronix as calibration Fixture No. 067-0608-00. When
open, the switch is simply 50 Ω to ground and settling is purely
a passive pulse decay and inherently flat. The low repetition rate
signal was captured on a digital oscilloscope after being
ꢀ kΩ
300 Ω
ꢀ±0 Ω
ꢀ kΩ
300 Ω
ꢀ±00 Ω
≈20 pF
≈ꢀ0 pF
≈± pF
−ꢀ0
amplified and clamped twice. The selection of plug-in for the
oscilloscope was made for minimum overload recovery.
Rev. F | Page ꢀ4 of 20
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