Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Help required in driving capacitive load using opamp?

Status
Not open for further replies.

hemnath

Advanced Member level 2
Joined
Jun 24, 2012
Messages
692
Helped
61
Reputation
120
Reaction score
57
Trophy points
1,308
Location
Chennai
Activity points
6,522
Hi,
I have the input signal of 1.8V at 600Khz ramp waveform.

Attached is the circuit used.

I want to replicate input waveform using LM6181 opamp.

When the capacitor value changes at the output of LM6181, the output waveform shape changes. How to avoid this. Please help
 

Attachments

  • Circuit.pdf
    5.6 KB · Views: 28

betwixt

Super Moderator
Staff member
Joined
Jul 4, 2009
Messages
14,931
Helped
4,874
Reputation
9,766
Reaction score
4,662
Trophy points
1,393
Location
Aberdyfi, West Wales, UK
Activity points
127,018
It's almost impossible to get an exact replica of the input shape while driving a capacitive load. You could try moving the feedback resistor to the other side of R18 so the changed wave shape is included in the feedback voltage.

Brian.
 

hemnath

Advanced Member level 2
Joined
Jun 24, 2012
Messages
692
Helped
61
Reputation
120
Reaction score
57
Trophy points
1,308
Location
Chennai
Activity points
6,522
moved the resistor as you suggested. The waveform shape is different. I would like to have the output waveform similar to the input waveform.

Any other help.

Thanks.

- - - Updated - - -

Also Please tell that Shall I use LM6181 to drive capacitive load. In the datasheet it says,

" The amplifier can directly drive upto 100 pF capacitive loads without oscillating and a
± 5 V and ±15 V 10-V signal into a 50-Ω or 75-Ω back-terminated coax cable system over the full industrial temperature
range."
Please confirm that this IC is better to use for capacitive loads? or should I look for another IC.
 

betwixt

Super Moderator
Staff member
Joined
Jul 4, 2009
Messages
14,931
Helped
4,874
Reputation
9,766
Reaction score
4,662
Trophy points
1,393
Location
Aberdyfi, West Wales, UK
Activity points
127,018
No IC, or any amplifier likes to drive a capacitive load. The combination of capacitance and drive impedance cause phase shifts that can make it unstable, that is why the data sheet spacifies maximum 100pF across the output. The LM6181 is really a cable driver, it is designed to work with a gain of 2 so it can drive a terminated 50 or 75 Ohm cable.

What waveform are you feeding it and what are you seeing at the output?
Are you taking into account the possibility of additional capacitve loading by the circuitry following it?
What are the variable components supposed to do?

Your solution, may be to use the LM6181 as a conventional amplifier and follow it with an emitter follower to isolate the capacitance from the amplifier output pin.

Brian.
 

BradtheRad

Super Moderator
Staff member
Joined
Apr 1, 2011
Messages
14,062
Helped
2,796
Reputation
5,592
Reaction score
2,705
Trophy points
1,393
Location
Minneapolis, Minnesota, USA
Activity points
104,892
When the ramp wave reaches the end, you have a sudden transition. The op amp must charge (or discharge) your output capacitor very quickly. Therefore the op amp needs to provide high current.

This is the same as having a fast time constant. Very low RC value. In other words your op amp output must be low resistance.

If it is high resistance, then your capacitor charges or discharges more slowly. You lose the sharp peak where the ramp transitions.
 

schmitt trigger

Advanced Member level 5
Joined
Apr 17, 2013
Messages
2,725
Helped
858
Reputation
1,720
Reaction score
845
Trophy points
1,393
Activity points
18,400
By "ramp waveform" do you mean sawtooth or triangle?

As Brad mentions, if it were a sawtooth, at that frequency it would require a discharge current that the opamp may not be able to provide. At least not in linear mode.

Assuming an ideal opamp, to discharge 1.8 volt with a 100 ohm limiting resistor requires a peak current of 18 mA. What does the LM6181 datasheet indicate?
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
19,728
Helped
4,344
Reputation
8,697
Reaction score
4,304
Trophy points
1,393
Activity points
130,573
Hi,

Please show us your input and output signal.
And add comments what you don´t like at the output signal.

Your negative supply has no capacitor. Please add one.
In general for a high precision signal processing you need a carefull layout and parts selection.
Don´t use long wires. Use resistors an capacitors for high frequency (no wirewound resistors...)
Mind that some scope probe (capacitance) may increase the distortion.
Proper ground connection is essential.

Klaus

- - - Updated - - -

Hi,

did you read datasheet:

chapter "7.5 Driving Capacitive Loads"?

Klaus
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top