OPA656 datasheet(11/25 Pages) NSC

11 / 25 page

OPA656

SBOS196G

www.ti.com

is peaked up over frequency by the diode source capaci-

tance, and can, in many cases, become the limiting factor to

input sensitivity. The key elements to the design are the

expected diode capacitance (CD) with the reverse bias volt-

age (–VB) applied, the desired transimpedance gain, RF, and

the GBP for the OPA656 (230MHz). Figure 3 shows a design

from a 25pF source capacitance diode through a 50k

Ω

transimpedance gain. With these 3 variables set (including

the parasitic input capacitance for the OPA656 added to CD),

the feedback capacitor value (CF) may be set to control the

frequency response.

If the total output noise is bandlimited to a frequency less

than the feedback pole frequency (1/RFCF), a very simple

expression for the equivalent input noise current can be

derived as:

EC F









+ (

)

Where:

iEQ = Equivalent input noise current if the output noise is

bandlimited to F < 1/(2

πR

FCD).

iN = Input current noise for the op amp inverting input.

eN = Input voltage noise for the op amp.

CD = Diode capacitance.

F = Bandlimiting frequency in Hz (usually a postfilter prior

to further signal processing).

4kT = 1.6E – 20J at 290

°K.

Evaluating this expression up to the feedback pole frequency

at 3.8MHz for the circuit of Figure 3, gives an equivalent input

noise current of 2.7pA/ Hz . This is much higher than the

1.3fA/ Hz for just the op amp itself. This result is being

dominated by the last term in the equivalent input noise

current expression. It is essential in this case to use a low

voltage noise op amp.

DESIGN-IN TOOLS

DEMONSTRATION FIXTURES

Two printed circuit boards (PCBs) are available to assist in

the initial evaluation of circuit performance using the OPA656

in its two package options. Both of these are offered free of

charge as unpopulated PCBs, delivered with a user's guide.

The summary information for these fixtures is shown in

Table I.

To achieve a maximally flat 2nd-order Butterworth frequency

response, the feedback pole should be set to:

)

(

))

ππ

GBP

Adding the common mode and differential mode input ca-

pacitance (0.7 + 2.8)pF to the 25pF diode source capaci-

tance of Figure 3, and targeting a 50k

Ω transimpedance gain

using the 230MHz GBP for the OPA656 will require a

feedback pole set to 3.8MHz. This will require a total feed-

back capacitance of 0.8pF. Typical surface-mount resistors

have a parasitic capacitance of 0.2pF leaving the required

0.6pF value shown in Figure 3 to get the required feedback

pole.

This will give an approximate –3dB bandwidth set by:

GBP

R C

−

The example of Figure 3 will give approximately 5.7MHz flat

bandwidth using the 0.6pF feedback compensation.

FIGURE 3. Wideband, Low-Noise, Transimpedance Amplifier.

50k

Ω

Supply Decoupling

Not Shown

25pF

OPA656

+5V

–5V

–V

O = ID RF

0.6pF

ORDERING

LITERATURE

PRODUCT

PACKAGE

NUMBER

OPA656U

SO-8

DEM-OPA-SO-1A

SBOU009

OPA656N

SOT23-5

DEM-OPA-SOT-1A

SBOU010

TABLE I. Demonstration Fixtures by Package.

The demonstration fixtures can be requested at the Texas

Instruments web site (www.ti.com) through the OPA656

product folder.

OPA656 데이터시트(PDF) 11 Page - National Semiconductor (TI)

OPA656 데이터시트(HTML) 11 Page - National Semiconductor (TI)

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

데이터시트 다운로드

링크 URL