Data Sheet

ADA4352-2

analog.com

Rev 0

36 of 47

APPLICATIONS INFORMATION

RC Tolerancing

The ADA4352-2’s internal RFXand CFX values come with the typical tolerancing for IC elements. The RFX values have

their nominal ±11% tolerance, while their associated CFX values are specified with ±20% tolerance. The RFX variation

is normally calibrated out to correct each of the gain slopes. The CFXvariation modifies the TIA frequency response

and changes the pulse responses at the TIA output (VOUT). Table 9 summarizes the specified RFX and CFXranges, and

the resulting feedback pole (P1) range.

Table 9. Specified RFXand CFX Tolerancing Ranges and Feedback Pole, P1

Nom RFX

Min RFX

Max RFX

Nom CFX

Min CFX

Max CFX

Max P1

Nom P1

Min P1

kΩ

kΩ

kΩ

pF

pF

pF

MHz

MHz

MHz

0.315

0.28

0.35

33

26.4

39.6

21.5

15.3

11.5

3.5

3.1

3.9

26.5

21.2

31.8

2.4

1.7

1.3

40.2

36

45

7

5.6

8.4

0.8

0.57

0.4

450.0

400.5

499.5

2.5

2

3

0.2

0.14

0.11

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1

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All these RFX selections have the same range of variation that can be calibrated out. The biggest impact of this RFX,

CFX tolerancing is for RF0 = 315 Ω, where the response shows peaking even with a relatively small assumed diode

source capacitance (CD) of 10 pF at the ADA4352-2’s input.

Adding an external RC filter of REXT = 200 Ω and CEXT = 180 pF at the TIA output, using 1% tolerance resistors and 2%

tolerance capacitors, significantly reduces peaking in the frequency response when using RF0 = 315 Ω. The F−3dB

frequencies of the higher gains RF1 to RF3 are already well below the external RC filter cutoff frequency. So, they are

not impacted by the external RC filter.

The Monte Carlo plot of Figure 75 shows both the ADA4352-2’s response variation due to internal tolerances of RF0 =

315 Ω and CF0 = 33 pF, and the subsequent improvement in peaking due to adding the external RC filter. Note that

the ADA4352-2 LTspice model that produces the plot of Figure 75 is configured with only the nominal internal

RFX||CFX values.

Figure 75. Monte Carlo of PGTIA Frequency Response for RF0 = 315Ω Nominal without (Black) and with External RC Filter

(Blue) with CD = 10 pF

55

50

45

40

10k

100k

1M

FREQUENCY (Hz)

10M

100M

CD = 10pF

REXT = 200Ω

CEXT = 180pF