Data Sheet

ADA4352-2

analog.com

Rev 0

39 of 47

Figure 79.

Simulated (Left) and Worst Case (Right) Frequency Response After External Filter for Various CD Values,

RF0 = 315 Ω.

Adding an RC filter at the output pin of the PGTIA with a roll off at 4.4 MHz results in the family of curves shown in

Figure 79. As CD increases, the increasing gain peaking of the PGTIA output nudges the gain upwards at frequencies

beyond the 4.4 MHz single pole roll off, causing an apparent bandwidth extension after the external filter.

For all gain settings higher than RF0 = 315 Ω, varying CD does not cause peaking in the frequency responses, as they

are much more bandlimited to begin with. The external RC filter simply bandlimits the ADC input and slightly

reduces integrated noise.

Figure 80.

Simulated Frequency Response for Various CD Values, RF1 = 3.5 kΩ

The next higher gain, the RF1 = 3.5 kΩ, CF1 = 26.5 pF nominal internal feedback setting, results in this family of

response curves (Figure 80) at the PGTIA output. Here, the F−3dB is increasing as the CD is changed from 5 pF to 25 pF,

and finally to 45 pF. This results from the frequency response poles moving from a simple 2-real pole response to a

higher quality factor (Q) 2nd order response, while remaining clustered around 2 MHz. Refer to the “TIA Design

Theory” in the ADA4351-2 data sheet for a more in-depth discussion of the TIA 2nd order frequency response,

including how Q and fo set the frequency response shape.

REXT = 200 Ω

CEXT = 180pF

FREQUENCY (Hz)

10k

100k

1M

10M

100M

40

45

50

55

CD = 5pF

CD = 25pF

CD = 45pF

REXT = 200 Ω

CEXT = 180pF

CD = 45pF

FREQUENCY (Hz)

10k

100k

1M

10M

100M

40

45

50

55

72

71

70

69

68

67

66

CD = 5pF

CD = 25pF

CD = 45pF

65

64

63

62

10k

100k

1M

FREQUENCY (Hz)

10M