Join

Sign
In with my.TI Login

Products

Applications

Tools
& Software

Support &
Community

Sample & Buy

About
TI

Sample
& Purchase Cart

Just this
area

Advanced

Support Forums

Blogs

Groups

Videos

简体中文

More
...

TI
Home » TI E2E
Community » Blogs
» On
Board with Bonnie » Those
darned op amp cap loads

Those darned op amp cap loads

On Board
with Bonnie

Subscribe to this Blog by Email

Subscribe

RSS for
posts

RSS
for comments

Tweet

Those darned op amp cap loads

Bonnie Baker
- WEBENCH Design Center

Jun 03 2014 15:56 PM

Comments 1

They say that if you are using your amplifier to drive a capacitive load
(Figure 1, CLOAD), a good rule of thumb is to isolate the
amplifier from the capacitor with a 50 or 100 Ohm resistor (RISO).
This additional resistor may stop your operational amplifier (op amp)
from oscillating.

Figure 1. An amplifier with a capacitive load may require a resistor
between the amplifier output and the load capacitor.

The use of 50 or 100 Ohms (RISO) may not work every time. A key
question to ask is, “What do I do if my CLOAD exceeds the recommended
data sheet op amp capacitive load value?”

If you can’t find any data sheet guidance or your load capacitance
(CLOAD) does exceed data sheet recommendations, the answer to this
question depends on the:

amplifier gain bandwidth product (GBWP or fU)

amplifier’s open-loop output resistance (RO)

capacitor load value (CLOAD)

The frequency versus gain graph in Figure 1 shows what happens to the
amplifier’s open loop gain curve when an RISO and CLOAD
are added at the output of your amplifier. If you use these three variables, you
will find the appropriate RISO value.

Here are rules when determining the value of RISO:

(Eq. 1)

(Eq. 2)

These rules ensure that the circuit is stable.

An appropriate application to apply this concept is where you are driving the
input to a SAR-ADC. In this case, you want the signal to settle within the
acquisition time (tACQ) of the converter. In Equation 3, K is an ADC
time-constant multiplier that provides half-LSB accuracy. For a 16-bit
converter, such as the ADS7886,
K equals 11.78.

(Eq. 3)

Let’s put these formulas to use. Working with the following parameters:

With the OPA365

fU = 50 MHz

R0 = 30 ohms

Gain = 1 V/V

With the ADS7886

tACQ = 300 nsec

CIN = 21 pF

CLOAD = 390 pF

According to the OPA365
data sheet, a load of 100 pF will product an overshoot of 30% (Figure 2).

Figure 2. OPA365 Overshoot
versus capacitive load

To solve this overshoot problem, equations 1, 2, and 3 can help.

With equation 1, RISO =>   3.33 Ohms

With equation 2, RISO => 30.97 Ohms

With equation 3, RISO  ~  61.96 Ohms

Given these three equations, RISO must be equal to 61.9 Ohms (0.1%
tolerance).

Some manufacturers include a stability versus capacitive load typical
performance curve in their data sheet. They may even provide a line in the
electrical performance table. All this information is very helpful, of course.
However, you can avoid amplifier oscillations or excessive overshoot with the
calculations used in this post.

References

For more information, download these datasheet: OPA365,
ADS7886

Baker, Bonnie, “Start
with the right op amp when driving SAR ADCs,” EDN, October 16, 2008

Baker, Bonnie, “Charge
your SAR-converter inputs,” EDN, May 11, 2006

$core_v2_language.FormatString($ti.GetResource('Blog_PostQuestionAnswerView_CommentsCountFormatString'),
$post.CommentCount)

Precision
DAC (=<10MSPS), Difference
Amplifier, Fully
Differential Amplifier, General
Purpose Amplifier, High
Output Current Amplifier (>=50mA), High
Supply Voltage Amplifier (>=30V), Instrumentation
Amplifier, Low
Input Bias Current Amp (<=10pA), Low
Noise Amp (<=10nV/rtHz), Low
Offset Voltage Amplifier (<=500uV), Low
Power Amp (<=500uA), Low
Supply Voltage Amplifier (<=2.7V), Precision
ADC (<=10MSPS), Precision
Amplifier, Zero
Drift Precision Amplifier, Integrated
Precision ADC and DAC, Precision
DAC with 4 to 20mA current output, ADS7886,
OPA365

Sign
in to Comment

Comments

Aleksandar
Markovic

Jun 16 2014 03:03 AM

What is the impact on stability and transfer gain if FB is taken after
those Riso resistor?

TI E2E™ Community

Support
Forums

Blogs

Videos

Groups

Site Support
& Feedback

Settings

TI E2E™ Community Groups

TI
University Program

Make the
Switch

Microcontroller
Projects

Motor Drive
& Control

Other Communities

Deyisupport

Designsomething.org

beagleboard.org

TI
on Element 14

TI
on TechXchangeSM

Other Technical & Support Resources

WEBENCH® Design Center

Product Information
Centers

Technical
Documents

TI Design Network

TI
Technical Articles

TI
Training

All content and materials on this site are provided "as is". TI and its
respective suppliers and providers of content make no representations about the
suitability of these materials for any purpose and disclaim all warranties and
conditions with regard to these materials, including but not limited to all
implied warranties and conditions of merchantability, fitness for a particular
purpose, title and non-infringement of any third party intellectual property
right. TI and its respective suppliers and providers of content make no
representations about the suitability of these materials for any purpose and
disclaim all warranties and conditions with respect to these materials. No
license, either express or implied, by estoppel or otherwise, is granted by TI.
Use of the information on this site may require a license from a third party, or
a license from TI.

Content on this site may contain or be subject to specific guidelines or
limitations on use. All postings and use of the content on this site are subject
to the Terms of
Use of the site; third parties using this content agree to abide by any
limitations or guidelines and to comply with the Terms
of Use of this site. TI, its suppliers and providers of content reserve the
right to make corrections, deletions, modifications, enhancements, improvements
and other changes to the content and materials, its products, programs and
services at any time or to move or discontinue any content, products, programs,
or services without notice.

Follow Us

TI Worldwide | Contact Us |
my.TI
Login |
Site Map | Corporate
Citizenship |
m.ti.com (Mobile Version)

TI is a global semiconductor design and manufacturing company. Innovate with
100,000+ analog ICs and
embedded processors, along with software, tools and
the industry’s largest sales/support staff.

© Copyright
1995-2013 Texas Instruments Incorporated. All rights reserved.
Trademarks |
Privacy
Policy | Terms
of Use