ADA4351-2ACPZ-R7 datasheet(33/36 Pages) AD

33 / 36 page

Data Sheet

ADA4351-2

APPLICATIONS INFORMATION

analog.com

Rev. 0 | 33 of 36

COMPENSATED AND NONCOMPENSATED

FREQUENCY RESPONSE

For any given source capacitance and desired TIA gain (RF), the

small signal frequency response may or may not be acceptable to

the application. Some response shape tuning is possible by adding

a compensation capacitor in parallel with RF. Figure 95 through

Figure 97, over a wide range of gains, show some typical response

shape trade-offs.

The lowest feedback (RF = 200 Ω) has the highest bandwidth

because the noise gain zero is relatively higher in frequency than

the higher gains with the same source capacitance. As previously

mentioned, at the lowest RF value, the output swing range is

reduced by the IR drop through the inside of the loop switch

impedance (approximately 0.7 V, see Figure 84). In each of the

conditions reported in Figure 95 through Figure 97, CIN = CD +

CSTRAY (refer to the TIA Design Theory section for definitions of

the terms). To get the total CS, add the internal input capacitances

(CCM + CDIFF) of 5.5 pF to CIN. Similarly, to get the total CF, add

the internal 3 pF to the CF,EXT shown in Figure 95 through Figure

97. For both of the 10 pF source CIN curves shown in Figure 95,

the noise gain zero and pole frequencies are beyond the 8.5 MHz

GBP; therefore, typical TIA analysis does not apply. The circuit with

CIN = 10 pF whose frequency response is shown in Figure 95 is

essentially operating as a unity-gain stage with a flat frequency

response with a 10 MHz f−3 dB.

The 100 pF CIN puts the noise gain zero just less than the GBP.

With no external CF,EXT, this zero gives a relatively low phase

margin and about 1.1 dB peaking because the noise gain pole is

much higher in frequency. Adding a 150 pF CFEXT moves the noise

gain zero down to 3 MHz with a noise gain pole at 5.2 MHz (see

Figure 95) giving a nicely overcompensated design with Q ≈ 0.62

and an f−3 dB near 4.5 MHz.

Figure 95. Small Signal Responses for RF = 200 Ω with a Range of Input CIN

(CD + CSTRAY) and External Feedback (CF,EXT) Capacitors

Going to a midrange feedback gain of 15 kΩ gives the family of

curves shown in Figure 96.

Figure 96. Small Signal Responses for RF = 15 kΩ with a Range of Input CIN

(CD + CSTRAY) and External Feedback (CF,EXT) Capacitors

The initial 10 pF source with only an internal 3 pF feedback

capacitor forms a noise gain zero below f0 (see the solid blue line

in Figure 96) but a feedback pole more than f0 giving the typical

peaked response shown by the solid blue curve shown in Figure

96. The peaking noise gain is crossing over the AOL curve with

slightly less than a 40 dB closure rate giving the approximate Q

= 1.1 and the 20log(21 kΩ/15 kΩ) = 2.9 dB peaking. Adding the

external 5.6 pF retunes the response shape to a controlled Q = 0.56

and f−3 dB ≈ 1.4 MHz, as shown by the dotted blue line shown in

Figure 96.

Testing an 100 pF external CIN, with only the internal 3 pF CF,INT,

again undercompensates the design (Q ≈ 2.7 with ≈9 dB peaking).

For this RF value, CF,EXT is required and the 16 pF shown in Figure

96 compensates this design back to Q = 0.62 with a f−3 dB = ≈730

kHz. Figure 96 is used to illustrate a typical TIA design flow and an

integrated noise analysis in those applications sections.

Using a large external RF (1 MΩ) results in the overcompensated

response shown in Figure 97 due to the internal 3 pF capacitor.

Figure 97. Small Signal Responses for RF = 1 MΩ with a Range of Input CIN

(CD + CSTRAY) Capacitors

ADA4351-2ACPZ-R7 数据表(PDF) 33 Page - Analog Devices

ADA4351-2ACPZ-R7 数据表(HTML) 33 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

数据表下载