LMH6611_14 Datasheet, PDF (32/42 Pages) National Semiconductor (TI) – LMH6611/LMH6612 Single Supply 345 MHz Rail-to-Rail Output Amplifiers

LMH6611, LMH6612
SNOSB00K – NOVEMBER 2007 – REVISED OCTOBER 2013
www.ti.com
Theoretical expressions for calculating the optimum value of C_F and the expected −3 dB bandwidth are:
C_T
C_F = _25RF(GBWP)
(7)
GBWP
f_{-3 dB = 25RFCT}
(8)
Equation 8 indicates that the −3 dB bandwidth of the TIA is inversely proportional to the feedback resistor.
Therefore, if the bandwidth is important then the best approach would be to have a moderate transimpedance
gain stage followed by a broadband voltage gain stage.
Table 4 shows the measurement results of the LMH6611 with different photodiodes having various capacitances
(C_PD) and a feedback resistance (R_F) of 1 kΩ.
Table 4. TIA (Figure 66) Compensation and Performance Results⁽¹⁾
C_PD
(pF)
C_T
(pF)
C_{F CAL}
(pF)
C_{F USED}
(pF)
f _{−3 dB CAL}
(MHz)
f _{−3 dB MEAS}
(MHz)
Peaking
(dB)
22
24
5.42
5.6
29.3
27.1
0.5
47
49
7.75
8
20.5
21
0.5
100
102
11.15
12
14.2
15.2
0.5
222
224
20.39
18
9.6
10.7
0.5
330
332
20.2
22
7.9
9
0.8
(1) GBWP = 130 MHz, C_T = C_PD + C_IN, C_IN = 2 pF, V_S = ±2.5V
Figure 79 shows the frequency response for the various photodiodes in Table 4.
3
0
-3
C_PD = 47 pF,
-6 _{CF = 8 pF}
-9
C_PD = 22 pF,
C_F = 5.6 pF
-12
-15
-18
-21
-24
-27
1M
C_PD = 100 pF,
C_F = 12 pF
C_PD = 222 pF,
C_F = 18 pF
C_PD = 330 pF,
C_F = 22 pF
10M
100M
FREQUENCY (Hz)
Figure 79. Frequency Response for Various Photodiode and Feedback Capacitors
When analyzing the noise at the output of the TIA, it is important to note that the various noise sources (that is,
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
pole (f_Z and f_P in Figure 78). The higher the values of R_F and C_T, the sooner the noise gain peaking starts and
therefore its contribution to the total output noise will be larger. It is advantageous to minimize C_IN by proper
choice of op amp or by applying a reverse bias across the diode but this will be at the expense of excess dark
current and noise.
32
Submit Documentation Feedback
Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LMH6611 LMH6612