LMH6644MAX datasheet(19/32 Pages) TI

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200 mV/DIV

20 ns/DIV

Vbias

200pF

Photodiode

100nF

2N3904

510:

1.8k:

1N4148

1k:

R11

910

R10

1k:

-1mAPP

5pF

1k:

VCC =

+5V

Rbias

Photodiode

Equivalent

Circuit

×100k:

Vout

+5V

SQRT

GBWP/(2SRF ˜ CIN)

fP =

CF =

SQRT

(CIN)/(2S ˜ GBWP ˜ RF)

LMH6642, LMH6643, LMH6644

www.ti.com

SNOS966P – MAY 2001 – REVISED MARCH 2013

No matter how low an Rf is selected, there is a need for Cf in order to stabilize the circuit. The reason for this is

that the Op Amp input capacitance and Q1 equivalent collector capacitance together (CIN) will cause additional

phase shift to the signal fed back to the inverting node. Cf will function as a zero in the feedback path counter-

acting the effect of the CIN and acting to stabilized the circuit. By proper selection of Cf such that the Op Amp

open loop gain is equal to the inverse of the feedback factor at that frequency, the response is optimized with a

theoretical 45° phase margin.

(1)

where GBWP is the Gain Bandwidth Product of the Op Amp

Optimized as such, the I-V converter will have a theoretical pole, fp, at:

(2)

With Op Amp input capacitance of 3pF and an estimate for Q1 output capacitance of about 3pF as well, CIN =

6pF. From the typical performance plots, LMH6642/6643 family GBWP is approximately 57MHz. Therefore, with

Rf = 1k, from Equation 1 and Equation 2 above.

Cf = ∼4.1pF and fp = 39MHz

Figure 63. Single Supply Photodiode I-V Converter

For this example, optimum Cf was empirically determined to be around 5pF. This time domain response is shown

in Figure 64 below showing about 9ns rise/fall times, corresponding to about 39MHz for fp. The overall supply

current from the +5V supply is around 5mA with no load.

Figure 64. Converter Step Response (1VPP, 20 ns/DIV)

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