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ADA4350

Data Sheet

Rev. B | Page 36 of 37

USING THE T NETWORK TO IMPLEMENT LARGE

FEEDBACK RESISTOR VALUES

Large feedback resistors (>1 MΩ) can cause the two following

issues in the transimpedance amplifier design:



If the parasitic capacitance of the feedback resistor exceeds

the optimal compensation value, it can significantly reduce

the TIA signal bandwidth.



If the required compensation capacitance is too low

(<1 pF), it is not practical to choose a feedback capacitor.

The T network (the RFx, R2, and R1 resistors) maintains the

transimpedance gain and signal bandwidth with a lower

feedback resistor and a resistive gain network, as shown in

Figure 70.

CFx

RFx

VOUT

TIA

IPHOTO

Figure 70. T Network

The relationship between the transimpedance VOUT/IPHOTO and

the T network resistors (RFx, R1, and R2) can be expressed as





















PHOTO

OUT

(10)

where:

VOUT is the output voltage of the TIA.

IPHOTO is the input photodiode current.

ZF = RFx/((RFx × CFx)s + 1), where RFx and CFx are the feedback

resistor and capacitor, respectively, of any of the chosen

transimpedance gain paths.

R1 and R2 are the T network gain resistors.

If ZF >> R2, the transimpedance equation is simplified to











 











PHOTO

OUT

)

(

Therefore, as compared to the standard TIA design, the T network

uses a feedback resistor value that is 1/(1 + R1/R2) smaller to

obtain the same transimpedance. This eliminates the concern of

the high parasitic capacitance associated with the large feedback

resistor. To maintain the same signal bandwidth (or same pole),

increase CF by a factor of 1 + R2/R1 to eliminate concerns of an

impractical small compensation capacitor.

As compared to a standard TIA design, the T network is noisier

because the dominant voltage noise density is amplified by the

gain factor 1 + R2/R1.

Figure 71 shows the ADA4350 configured as a 1 MΩ trans-

impedance path and its T network equivalent. Figure 72 compares

the performance of the 1 MΩ path and the equivalent T network

with and without compensation capacitors.

3.3pF

100kΩ

0.5pF

1MΩ

VOUT

111Ω

1kΩ

TIA

IPHOTO

CD = 91pF

Figure 71. 1 MΩ Transimpedance Path and its Equivalent T Network

FREQUENCY (Hz)

10k

100k

10M

10k

100k

10M

VS = ±5V, DVDD = +5V

CD = 91pF

RF = 1MΩ

1MΩ T NETWORK EQUIVALENT

RF = 1MΩ, CF = 500fF

1MΩ T NETWORK EQUIVALENT, CF = 3.3pF

Figure 72. Comparing the 1 MΩ Transimpedance Path and

T Network Performance

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