LMH6601MG NSC [National Semiconductor], LMH6601MG Datasheet - Page 24

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

www.national.com
Application Information
With the LMH6601 input bias current in the femto-amperes
range, even large values of gain (R
output error term appreciably. This allows circuit operation to
a lower light intensity level which is always of special impor-
tance in these applications. Most photo-diodes have a rela-
tively large capacitance (C
a photo-diode designed for higher sensitivity to light because
of its larger area. Some applications may run the photodiode
with a reverse bias in order to reduce its capacitance with the
disadvantage of increased contributions from both dark cur-
rent and noise current. Figure 14 shows a typical photodiode
capacitance plot vs. reverse bias for reference.
The diode capacitance (C
of the LMH6601 (C
circuit and how it is compensated. With large transimped-
ance gain values (R
the amplifier inverting input (C
R
Figure 15). If left untreated, at higher frequencies where NG
equals the open loop transfer function there will be excess
phase shift around the loop (approaching 180˚) and there-
fore, the circuit could be unstable. This is illustrated in Figure
15.
F
FIGURE 14. Typical Capacitance vs. Reverse Bias
to create a zero in the Noise Gain (NG) function (see
(Source: OSI Optoelectronics)
A
) has a bearing on the stability of this
F
), the total combined capacitance on
D
D
) along with the input capacitance
) which would be even larger for
IN
= C
D
F
+ C
) do not increase the
A
) will work against
(Continued)
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Figure 15 shows that placing a capacitor, C
value, across R
For optimum performance, this capacitor is usually picked so
that NG is equal to the op amp’s open loop gain at f
will cause a “flattening” of the NG slope beyond the point of
intercept of the two plots (open loop gain and NG) and will
results in a Phase Margin (PM) of 45˚ assuming f
at least a decade apart. This is because at the point of
intercept, the NG pole at f
contribution which leaves 45˚ of PM. For reference, Figure
15 also shows the transimpedance gain (I-V (Ω))
Here is the theoretical expression for the optimum C
and the expected −3 dB bandwidth:
Table 3, below, lists the results, along with the assumptions
and conditions, of testing the LMH6601 with various photo-
diodes having different capacitances (C
ance gain (R
FIGURE 15. Transimpedance Amplifier Graphical
Stability Analysis and Compensation
F
) of 10 kΩ.
F
will create a pole in the NG function at f
P
will have a 45˚ phase lead
D
) at a transimped-
F
, with the proper
P
and f
F
P
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. This
value
Z
(10)
are
(9)
P
.