LMH6601MG NSC [National Semiconductor], LMH6601MG Datasheet - Page 25

• Manufacturer Part Number: LMH6601MG
• Description: 250 MHz, 2.4V CMOS Op Amp with Shutdown
• Manufacturer: NSC [National Semiconductor]

Application Information
TRANSIMPEDANCE AMPLIFIER NOISE
CONSIDERATIONS
When analyzing the noise at the output of the I-V converter,
it is important to note that the various noise sources (i.e. op
amp noise voltage, feedback resistor thermal noise, input
noise current, photodiode noise current) do not all operate
over the same frequency band. Therefore, when the noise at
the output is calculated, this should be taken into account.
The op amp noise voltage will be gained up in the region
between the noise gain’s “zero” and its “pole” (f
Figure 15). The higher the values of R
the noise gain peaking starts and therefore its contribution to
the total output noise would be larger. It is obvious to note
that it is advantageous to minimize C
choice of op amp, by applying a reverse bias across the
diode at the expense of excess dark current and noise).
However, most low noise op amps have a higher input
capacitance compared to ordinary op amps. This is due to
the low noise op amp’s larger input stage.
OTHER APPLICATIONS
FIGURE 16. Charge Preamplifier Taking Advantage of
LMH6601’s Femto-Ampere Range Input Bias Current
(pF)
500
C
10
50
D
TABLE 3. Transimpedance Amplifier Figure 13 Compensation and Performance Results
(pF)
502
C
12
52
IN
GBWP = 155 MHz
C
V
A
S
= 2 pF
C
= 5V
F
_Calculated
(pF)
1.1
2.3
7.2
F
and C
IN
(Continued)
(e.g. by proper
IN
, the sooner
z
and f
C
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F
(pF)
used
p
1
3
8
in
25
CAPACITIVE LOAD
The LMH6601 can drive a capacitive load of up to 1000 pF
with correct isolation and compensation. Figure 17 illustrates
the in-loop compensation technique to drive a large capaci-
tive load.
When driving a high capacitive load, an isolation resistor
(R
output and the capacitive load to provide isolation and to
avoid oscillations. A small value capacitor (C
between the op amp output and the inverting input as shown
such that this capacitor becomes the dominant feedback
path at higher frequency. Together these components allow
heavy capacitive loading while keeping the loop stable.
There are few factors which affect the driving capability of
the op amp:
Table 4 shows the measured step response for various
values of load capacitors (C
back resistor (C
= 2 kΩ:
• Op amp internal architecture
• Closed loop gain and output capacitor loading
* Response limited by input step generator rise time of 5 ns
Calculated (MHz)
FIGURE 17. In-Loop Compensation Circuit for Driving
TABLE 4. LMH6601 Step Response Summary for the
S
) should be connected in series between the op amp
(pF)
300
500
910
110
C
10
50
−3 dB BW
L
14
7
2
192
(Ω)
R
47
80
0
0
6
F
S
a Heavy Capacitive Load
) with gain of +2 (R
Circuit of Figure 17
Measured (MHz)
(pF)
−3 dB BW
C
10
10
10
1
1
1
F
L
7.0
2.5
15
), series resistor (R
t
rise
F
(ns)
10
12
33
65
6*
7*
= R
/ t
fall
Step Response
G
Overshoot (%)
= 604Ω) and R
F
Overshoot
) is inserted
S
www.national.com
) and feed-
6
4
9
(%)
16
20
10
10
8
6
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L