Generate positive linear voltage ramp from single sided supply

[Steve]

I need to generate a positive going linear voltage ramp using a single
sided positive power supply. 0 to 100 mV needed but higher voltages
could be divided down. Variable and relatively long ramp times
between 1 and 60 seconds. It should hold its maximum value once the
ramp is done and be resettable to 0 V and held there by a switch until
the switch is activated again to start another single cycle ramp and
hold. Any way to do this with a single supply op amp such as the
OPA340 ?

 

[Rich Grise]

I need to generate a positive going linear voltage ramp using a single
sided positive power supply. 0 to 100 mV needed but higher voltages could
be divided down. Variable and relatively long ramp times between 1 and 60
seconds. It should hold its maximum value once the ramp is done and be
resettable to 0 V and held there by a switch until the switch is activated
again to start another single cycle ramp and hold. Any way to do this
with a single supply op amp such as the OPA340 ?

You don't even need an opamp. Just use a counter and any ol' DAC.

Have Fun!
Rich

 

[Spehro Pefhany]

I need to generate a positive going linear voltage ramp using a single
sided positive power supply. 0 to 100 mV needed but higher voltages
could be divided down. Variable and relatively long ramp times
between 1 and 60 seconds. It should hold its maximum value once the
ramp is done and be resettable to 0 V and held there by a switch until
the switch is activated again to start another single cycle ramp and
hold. Any way to do this with a single supply op amp such as the
OPA340 ?

Easy with a dual CMOS single-supply op-amp. A transistor and one
op-amp (and maybe a voltage reference) to form a constant current
source charging a cap (with other end of the cap grounded), and a gain
of 1 buffer. Definitely use as much of the supply voltage as
reasonable and divide it down. Another transistor to reset the ramp.
Vary the source resistor in the current-source transistor to change
the current (ramp time will be proportional to resistance)

Using a 10uF low-leakage capacitor, and going to say, 3V, you would
need 0.5uA for 60s, so a 0-1M rheostat with 500mV across it will give
you 0-60seconds for a ramp from 0-3V. Then divide it 30:1 down.

If you plan on a wide range of ramp times, modify the current source
slightly so that it shuts down when the RESET signal is active.

 

[Spehro Pefhany]

You don't even need an opamp. Just use a counter and any ol' DAC.

Have Fun!
Rich

Or a microcontroller, and use PWM. Or if you insist on 70s technology,
a digital comparator (2 or 3 74HC85 etc) and a free running counter
(eg. 10MHz) to compare with your ramp counter. Use the '>' output and
low-pass filter it.

 

[Fred Bartoli]

Spehro Pefhany a écrit :

Or a microcontroller, and use PWM. Or if you insist on 70s technology,
a digital comparator (2 or 3 74HC85 etc) and a free running counter
(eg. 10MHz) to compare with your ramp counter. Use the '>' output and
low-pass filter it.

Nah, a pot with a stepper and a good flywheel...

 

[Steve]

The ramp slows down a lot near its end in this approach due to the op
amp having reached its maximum output voltage (the supply rail) and
thus the capacitor charging current decreases and is no longer
constant.
"Instrumentation amp makes noninverting integrator"
http://www.edn.com/contents/images/91902di.pdf

The Howland Integrator works though. Just one single supply op amp, 4
resistors and a capacitor. 10 uF polypropylene, 1 M ohm R's and 1 V
or so creates tens of seconds ramp times.
Fig 7 in
http://www.national.com/an/AN/AN-1515.pdf
Also called the "Deboo" Integrator For Unipolar Noninverting Designs
http://electronicdesign.com/Articles/Index.cfm?AD=1&AD=1&ArticleID=1633

 

[Steve]

The 10 uF metallized polyproplyene that I have is 1 x 1 inch

 

[Steve]

That's why I mentioned the OPA340. It will go to within 1 mV of
positive and negative supply rails (with 100 K load). I have measured
its output at < 1 mV when it should be at 0.
Have a look at its data sheet for more info (Fig 15 for example) :
http://focus.ti.com/lit/ds/symlink/opa2340.pdf

 

[Tim Williams]

10uF tantalums fit in, geez, 1206 or smaller? Timing is one of the few
applications where tantalums [probably] won't explode in. ;-)

Tim

 

[Spehro Pefhany]

Current source (like, two PNP transistors in a current mirror) feeding
a capacitor to ground will do it. Clamp the ramp at some
convenient point (takes one diode and an op amp), and
buffer the output with an op amp, then attenuate.

No op amp I know of is really comfortable at 100 mV from the negative
supply rail. 'single supply' op amp means the INPUTS can be
near the rail, you want the OUTPUT to be near the rail.
Things get dicey.
My advice, is don't hold too tight to the 'gotta-use-a-single-supply'
requirement.

Yeah, but if he's dividing it down from (say) 3V, then the op-amp just
needs to be happy from (say) 150mV to 3V to give a ramp from 5mV to
100mV.


Best regards,
Spehro Pefhany

 

[krw]

10uF tantalums fit in, geez, 1206 or smaller? Timing is one of the few
applications where tantalums [probably] won't explode in. ;-)

Tim

Tantalums do no tnormally explode, I have used many and never had one explode.
Some have been on for > 10 years.
And they do not dry out like some electrolitics.
Give much better RF decoupling too.
Just do not reverse-connect them.

No matter what you do, some percentage *will* be inserted backwards.
Some are fused for this reason. Some radials had three or four leads
so they couldn't be inserted backwards. ;-)

 

[Rich Grise]

Technically, you would be generating a stair step signal, not a smooth
ramp. However, 60 seconds is a bit long for an analog solution. It depends
on what matters in the signal, i.e. smoothness or timing.

With 24 bits of counter and DAC (or however big they make DACs these days;
in the 1980's, they had 12 - do they go up to 24 yet?) you could make it
pretty darn smooth, I'd think.

Use synchronous counters, of course. ;-)

Cheers!
Rich

 

[Paul Keinanen]

With 24 bits of counter and DAC (or however big they make DACs these days;
in the 1980's, they had 12 - do they go up to 24 yet?) you could make it
pretty darn smooth, I'd think.

In addition, use some RC filtering after the DAC.

If the fall time is critical, use a digital output to reset the
capacitor voltage.

Even with a low bit count DAC, one or two digital signals could be
used to switch out part of the series resistance in the RC filter
after the DAC.

 

[JosephKK]

Spehro Pefhany a écrit :

Nah, a pot with a stepper and a good flywheel...

Now that is retro. About 30 years ago we used a gear motor to ramp a
400 V power supply.