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LMH6601_14 Datasheet, PDF (28/37 Pages) National Semiconductor (TI) – LMH6601/LMH6601Q 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown
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LMH6601, LMH6601-Q1
SNOSAK9E – JUNE 2006 – REVISED MARCH 2013
www.ti.com
Figure 61 shows that placing a capacitor, CF, with the proper value, across RF will create a pole in the NG
function at fP. For optimum performance, this capacitor is usually picked so that NG is equal to the op amp's
open loop gain at fP. This 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 fP and fZ are at least a decade
apart. This is because at the point of intercept, the NG pole at fP will have a 45° phase lead contribution which
leaves 45° of PM. For reference, Figure 61 also shows the transimpedance gain (I-V (Ω))
Here is the theoretical expression for the optimum CF value and the expected −3 dB bandwidth:
CF =
CIN
2S(GBWP)RF
(10)
GBWP
f-3 dB # 2SRFCIN
(11)
Table 3, below, lists the results, along with the assumptions and conditions, of testing the LMH6601 with various
photodiodes having different capacitances (CD) at a transimpedance gain (RF) of 10 kΩ.
Table 3. Transimpedance Amplifier Figure 59 Compensation and Performance Results
CD
CIN
CF_Calculated
CF used
−3 dB BW
−3 dB BW
Step Response
(pF)
(pF)
(pF)
(pF)
Calculated (MHz) Measured (MHz)
Overshoot (%)
10
12
1.1
1
14
15
6
50
52
2.3
3
7
7.0
4
500
502
7.2
8
2
2.5
9
CA = 2 pF GBWP = 155 MHz VS = 5V
(12)
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” (fz and fp
in Figure 61). The higher the values of RF and CIN, the sooner 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 CIN
(e.g. by proper 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.
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