LMH6601Q NSC [National Semiconductor], LMH6601Q Datasheet - Page 24

• Manufacturer Part Number: LMH6601Q
• Description: 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown
• Manufacturer: NSC [National Semiconductor]

www.national.com
The diode capacitance (C
of the LMH6601 (C
cuit and how it is compensated. With large transimpedance
gain values (R
plifier inverting input (C
create a zero in the Noise Gain (NG) function (see Figure
14). 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 therefore, the circuit
could be unstable. This is illustrated in Figure 14.
FIGURE 14. Transimpedance Amplifier Graphical
(pF)
500
C
10
50
D
Stability Analysis and Compensation
F
), the total combined capacitance on the am-
TABLE 3. Transimpedance Amplifier Figure 12 Compensation and Performance Results
(pF)
502
C
12
52
A
IN
) has a bearing on the stability of this cir-
GBWP = 155 MHz
IN
C
D
= C
V
) along with the input capacitance
A
S
= 2 pF
C
= 5V
D
F
+ C
_Calculated
(pF)
A
1.1
2.3
7.2
) will work against R
C
20136460
F
(pF)
used
F
1
3
8
to
24
Figure 14 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
cause a “flattening” of the NG slope beyond the point of in-
tercept 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 inter-
cept, the NG pole at f
which leaves 45° of PM. For reference, Figure 14 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 photodi-
odes having different capacitances (C
gain (R
Calculated (MHz)
−3 dB BW
F
) of 10 kΩ.
14
7
2
F
will create a pole in the NG function at f
P
Measured (MHz)
will have a 45° phase lead contribution
−3 dB BW
7.0
2.5
15
D
) at a transimpedance
Step Response
Overshoot (%)
F
, with the proper
P
6
4
9
P
and f
. This will
F
value
Z
(10)
are
(9)
P
.