OPA875 datasheet(11/18 Pages) TI

11 / 18 page

www.ti.com

OPERATING SUGGESTIONS

DRIVING CAPACITIVE LOADS

DESIGN-IN TOOLS

DEMONSTRATION FIXTURES

MACROMODELS AND APPLICATIONS

DC ACCURACY

OPA875

SBOS340 – DECEMBER 2006

Results for n varying from 2 to 6 are given in

Table 2.

Table 2. Series Resistance versus Number of

Parallel Outputs

One of the most demanding, yet very common load

conditions is capacitive loading. Often, the capacitive

NUMBER OF OPA875s

RO (Ω)

load is the input of an ADC—including additional

external capacitance that may be recommended to

63.94

improve ADC linearity. A high-speed device such as

59.49

the OPA875 can be very susceptible to decreased

55.59

stability and closed-loop response peaking when a

capacitive load is placed directly on the output pin.

52.15

When the device open-loop output resistance is

considered,

this

capacitive

load

introduces

The two major limitations of this circuit are the device

additional pole in the signal path that can decrease

requirements for each OPA875 and the acceptable

the phase margin. Several external solutions to this

return loss because of the mismatch between the

problem have been suggested. When the primary

load (75

Ω) and the matching resistor.

considerations are frequency response flatness,

pulse response fidelity, and/or distortion, the simplest

and most effective solution is to isolate the capacitive

load from the feedback loop by inserting a series

isolation resistor between the amplifier output and

the capacitive load. This isolation resistor does not

A printed circuit board (PCB) is available to assist in

eliminate the pole from the loop response, but rather

the initial evaluation of circuit performance using the

shifts it and adds a zero at a higher frequency. The

OPA875. The fixture is offered free of charge as an

additional zero acts to cancel the phase lag from the

unpopulated PCB, delivered with a user's guide. The

capacitive load pole, thus increasing the phase

summary information for this fixture is shown in

margin and improving stability.

Table 3.

The Typical Characteristics show the recommended

Table 3. OPA875 Demonstration Fixture

RS versus capacitive load and the resulting

frequency response at the load; see Figure 5.

ORDERING

LITERATURE

PRODUCT

PACKAGE

NUMBER

Parasitic capacitive loads greater than 2pF can begin

OPA875IDGK

MSOP-8

DEM-TIV-MSOP-1A

SBOU044

to degrade the performance of the OPA875. Long

PCB traces, unmatched cables, and connections to

OPA875ID

SO-8

DEM-TIV-SO-1A

SBOU045

multiple devices can easily cause this value to be

The demonstration fixture can be requested at the

exceeded. Always consider this effect carefully, and

Texas Instruments web site at (www.ti.com) through

add the recommended series resistor as close as

the OPA875 product folder.

possible to the OPA875 output pin (see the Board

Layout Guidelines section).

SUPPORT

Computer simulation of circuit performance using

The OPA875 offers excellent DC signal accuracy.

SPICE

often

useful

when

analyzing

the

Parameters that influence the output DC offset

performance of analog circuits and systems. This is

voltage are:

particularly true for video and RF amplifier circuits

• Output offset voltage

where parasitic capacitance and inductance can

• Input bias current

have a major effect on circuit performance. A SPICE

• Gain error

model for the OPA875 is available through the Texas

• Power-supply rejection ratio

Instruments web site at www.ti.com. These models

do a good job of predicting small-signal AC and

• Temperature

transient performance under a wide variety of

operating conditions. They do not do as well in

predicting

the

harmonic

distortion

dG/dP

characteristics. These models do not attempt to

distinguish between the package types in their

small-signal AC performance.

Submit Documentation Feedback

OPA875 Datasheet(PDF) 11 Page - Texas Instruments

OPA875 Datasheet(HTML) 11 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL