LMH6624_16 Datasheet, PDF (24/38 Pages) Texas Instruments – LMH6624 and LMH6626 Single/Dual Ultra Low Noise Wideband Operational Amplifier

LMH6624, LMH6626
SNOSA42G – NOVEMBER 2002 – REVISED DECEMBER 2014
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Typical Application (continued)
8.2.1 Design Requirements
Figure 58 shows the Noise Gain (NG) and transfer function (I-V Gain). As with most Transimpedance amplifiers,
it is required to compensate for the additional phase lag (Noise Gain zero at f_Z) created by the total input
capacitance: C_D (diode capacitance) + C_CM (LMH6624 CM input capacitance) + C_DIFF (LMH6624 DIFF input
capacitance) looking into R_F. This is accomplished by placing C_F across R_F to create enough phase lead (Noise
Gain pole at f_P) to stabilize the loop.
OP AMP OPEN
LOOP GAIN
I-V GAIN (:)
NOISE GAIN (NG)
1 + sR_F (C_IN + C_F)
1 + sR_FC_F
1 + ^CIN
C_F
0 dB
FREQUENCY _{fz #} 1
2SR_FC_IN
1
GBWP
f_P = _2SRFCF
Figure 58. Transimpedance Amplifier Noise Gain and Transfer Function
8.2.2 Detailed Design Procedure
The optimum value of C_F is given by Equation 8 resulting in the I-V -3dB bandwidth shown in Equation 9, or
around 124 MHz in this case, assuming GBWP = 1.5 GHz, C_CM (LMH6624 CM input capacitance) = 0.9 pF, and
C_DIFF (LMH6624 DIFF input capacitance) = 2 pF. This C_F value is a “starting point” and C_F needs to be tuned for
the particular application as it is often less than 1 pF and thus is easily affected by board parasitics.
Optimum C_F Value:
C_F =
C_IN
25(GBWP)R_F
(8)
Resulting -3dB Bandwidth:
f _{- 3 dB} 
GBWP
25R_F C_IN
(9)
Equation 10 provides the total input current noise density (i_ni) equation for the basic Transimpedance
configuration and is plotted against feedback resistance (R_F) showing all contributing noise sources in Figure 59.
The plot indicates the expected total equivalent input current noise density (i_ni) for a given feedback resistance
(R_F). This is depicted in the schematic of Figure 60 where total equivalent current noise density (i_ni) is shown at
the input of a noiseless amplifier and noiseless feedback resistor (R_F). The total equivalent output voltage noise
density (e_no) is i_ni*R_F. Noise Equation for Transimpedance Amplifier:
(10)
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