Data Sheet

ADA4352-2

analog.com

Rev 0

38 of 47

Frequency Response Variation with Diode Capacitance CD

External diode capacitance (CD) values interact with the ADA4352-2’s various internal feedback resistance and

capacitance pairs (RFX||CFX) to give a range of frequency response curves. The lowest (315 Ω) RF0 value, which results

in a F-3dB bandwidth greater than 10 MHz, shows increased peaking in the frequency response as the CD increases

from 5 pF to 25 pF, and finally to 45 pF. This resulting response is normally bandlimited by the typically <5 MHz RC

filter following the PGTIA into a SAR ADC (see RC Tolerancing and Effect of External RC Filter). In all cases, it is

important to remember the ADA4352-2 adds a total of 5.5 pF parasitic input common-mode and differential

capacitance (CCM + CDIFF) to CD.

The family of output response curves in the left half of Figure 78 shows the effect of changing the value of CD to 5 pF,

25 pF, or 45 pF at the RF0 = 315 Ω gain setting. As the diode capacitance is increased, the frequency response

exhibits more peaking and the corresponding F−3dB point moves slightly lower in frequency.

Figure 78.

Simulated (Left) and Measured (Right) Frequency Response at VOUT for Various CD Values, RF0 = 315 Ω.

Measured Curve with CD = 47 pF Shown for Worst Case Peaking Comparison

The gains RF0 through RF3 are displayed in dBΩ in frequency response plots. Table 10 provides the conversions from

linear gains to their logarithmic equivalents to clarify the frequency response plots in this section.

Table 10. Nominal RFX and Equivalent Logarithmic Gain for All Gain Settings of ADA4352-2

Nominal RFX Value (Ω)

Nominal RFX Value (dBΩ)

Gain Step (dBΩ)

Internal CFX Value (pF)

315

50

NA

33

3.5k

71

21

26

40.2k

92

21

7.0

450k

113

21

2.5

The following curves in Figure 79 to Figure 82, which show how the frequency response changes with CD, are

generated in LTspice.

FREQUENCY (Hz)

10k

100k

1M

10M 20M

40

45

50

55

CD = 5pF

CD = 25pF

CD = 45pF

CD = 47pF

FREQUENCY (Hz)

10k

100k

1M

10M 20M

40

45

50

55