J
Jim Thompson
I'll let it, if you'll measure it ;-)
...Jim Thompson
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I'll let it, if you'll measure it ;-)
...Jim Thompson
W
Winfield Hill
Walter Harley wrote...
You don't want current sources and a capacitor, instead think
about using a capacitor in an opamp integrator configuration.
Go ahead and use a low-cost chopper amp, and use low voltages
to a modest-sized integrator resistor, trimpot-controlling the
voltage to this resistor with conventional low-value stuff.
You can create a slew-lmited circuit that goes from one voltage
to another, as requested externally, but at the slow slew rate.
You don't want current sources and a capacitor, instead think
about using a capacitor in an opamp integrator configuration.
Go ahead and use a low-cost chopper amp, and use low voltages
to a modest-sized integrator resistor, trimpot-controlling the
voltage to this resistor with conventional low-value stuff.
You can create a slew-lmited circuit that goes from one voltage
to another, as requested externally, but at the slow slew rate.
J
Jim Thompson
That's odd. They're located in Massachusetts. In fact, I vaguely
recall buying a few of their chips about a year ago... Circuit
Specialists ???
They're not very electrically conductive, but they do BURN ;-)
Using a DAC doesn't require a uC. Use 555's, or something similar,
plus up-down counters.
...Jim Thompson
W
Walter Harley
"Fred Bartoli"
It's very simple! As you say, it's not very accurate, but that's not really
a big problem for me. It's a little awkward to adjust over a large range,
too; for any particular value of R, the range of adjustment (by changing the
10M resistor) is limited by the transistor's beta, because of the base
current needed to switch the current through R to ground. But I only need
an adjustment range of 1:20; this suffices. To get down to 50nA it needs a
10MEG trimpot, which I can't find, but I could double up my capacitance
(and/or scale up my voltage) and work with 100nA to 2uA.
It's very simple! As you say, it's not very accurate, but that's not really
a big problem for me. It's a little awkward to adjust over a large range,
too; for any particular value of R, the range of adjustment (by changing the
10M resistor) is limited by the transistor's beta, because of the base
current needed to switch the current through R to ground. But I only need
an adjustment range of 1:20; this suffices. To get down to 50nA it needs a
10MEG trimpot, which I can't find, but I could double up my capacitance
(and/or scale up my voltage) and work with 100nA to 2uA.
W
Walter Harley
Winfield Hill said:
How do I clamp the voltage across the capacitor?
It won't do to just clamp the output of the circuit, because then it
wouldn't immediately start ramping down when the logic changes.
One end of the capacitor is at ground (the inverting input of the opamp),
nothing to clamp there. The other end is at the opamp output, a
low-impedance point so no point clamping there either.
I could put a resistor in series with the opamp output, thus making it a
relatively higher-impedance point that I could then clamp. Since it would
be inside the feedback loop, it shouldn't affect the integrator. Like this:
--/\/\/------o-----||---------.
| |
'-|\ |
| >--/\/\/----o--- OUTPUT (clamped to 0V < Vo < Vmax)
,-|/
|
===
As long as the clamps are low leakage, and the output is driving a very high
impedance (another opamp input), this should work. Right?
By the way, thanks very much, everyone!
-walter
J
Jim Thompson
Walter,
Could you repeat your desired voltage limits/bounds? I don't think I
got all the thread. Also, what power supplies do you have available?
Thanks!
...Jim Thompson
J
Jim Thompson
On Fri, 16 Sep 2005 17:26:47 -0700, "Walter Harley"
[snip]
If you clamp the output as shown above, the OpAmp output will rail,
then the summing node will wander off somewhere, giving the same kind
of unknown-starting-point problem.
...Jim Thompson
[snip]
If you clamp the output as shown above, the OpAmp output will rail,
then the summing node will wander off somewhere, giving the same kind
of unknown-starting-point problem.
...Jim Thompson
F
Fred Bloggs
Jim said:
This is a simple 555 circuit with simple frequency control- I got 300
seconds with the LMC555 and Radio Shack 0.22u 50V films with less than
+/-5% variation for -15oC to 70oC, first time out. Thousands of little
ramps+ plateaus -should be good enough for some audio kluge and he can
buffer with the TL082 another one still available from Radio Shack.
W
Walter Harley
Jim Thompson said:
The thread is getting pretty spread out... there are a lot of ways to skin a
cat.
Goal is to make a linear voltage ramp from 0v to 1v, and back down, with
separate time constants for up and down, each adjustable by trimpot from
0.1s to 5s. The voltage could be higher; I can always divide it down, e.g.,
ramp 0 to 5V, and then divide by five.
I can make whatever power supplies I need, but I will be starting with
+/-15V (for the opamps) and perhaps +/-7.5V (in the event that I use a
CD4066 switch).
This is a one-off, so parts cost is not a big issue; important aspects are
simplicity, reliability, field serviceability, and low parts count (the
latter just so as to minimize assembly time and physical size).
Linearity should probably be no worse than, say, 20%, and the ramp time
(once set) should not vary more than 20% over a time period of several years
and over a temperature range of 20C to 35C.
***
Jim, you also said:
Oh, duh. Dunno what I was thinking. The current through the resistor has
to come from somewhere, and that somewhere has to be through the capacitor,
clamps or no. The only way to clamp the capacitor voltage is to prevent
current from flowing through it. Sorry, thinking out loud.
W
Winfield Hill
Walter Harley wrote...
Here's one way, with empirically-determined setpoint voltages:
.. ,----- setpoint - 0.4V
.. b
.. ,------c e -,
.. | |
.. --/\/\/------+--+----||-------+
.. | |
.. '-|\ |
.. | >--/\/\/----o--- OUTPUT
.. ,-|/
.. |
.. ===
If you want more control over the clamping voltage, it's necessary
to define the clamping-transistor's current, which is easily done
with a diode plus a sinking current (e.g., a resistor to V-):
.. ,----- setpoint - ~0.4V
.. b
.. ,--|<|--+----c e -, pnp
.. | | |
.. | sink R (to stabilize the loop)
.. | |
.. --/\/\/---+--+---------||-----+
.. | |
.. '-|\ |
.. | >--/\/\/----+--- OUTPUT
.. ,-|/
.. |
.. ===
Here's one way, with empirically-determined setpoint voltages:
.. ,----- setpoint - 0.4V
.. b
.. ,------c e -,
.. | |
.. --/\/\/------+--+----||-------+
.. | |
.. '-|\ |
.. | >--/\/\/----o--- OUTPUT
.. ,-|/
.. |
.. ===
If you want more control over the clamping voltage, it's necessary
to define the clamping-transistor's current, which is easily done
with a diode plus a sinking current (e.g., a resistor to V-):
.. ,----- setpoint - ~0.4V
.. b
.. ,--|<|--+----c e -, pnp
.. | | |
.. | sink R (to stabilize the loop)
.. | |
.. --/\/\/---+--+---------||-----+
.. | |
.. '-|\ |
.. | >--/\/\/----+--- OUTPUT
.. ,-|/
.. |
.. ===
F
Fred Bloggs
Winfield said:
The OP seems to be having problems recognizing his requirements, but
this is just a bog standard slew-rate controlled amplifier with three
input parameters: final value, sr(+), and sr(-). No inaccurate clamping
required and very uncomplicated, like so:
View in a fixed-width font such as Courier.
..
..
.. +----------------[R]--------------+
.. | |
.. | |
.. | |
.. | | |
.. | - |
.. | ||| |
.. | V ---> |V| BCS1 |
.. | SR1 ||| |
.. | - |
.. Vin>--[R]-+--|+\ | C |
.. | >--+--|<|-+-|>|--+----||----+--->Vout
.. +-|-/ | | |
.. | | | |
.. REF | | |
.. +--|>|-+-|<|--+-----|-\ |
.. | | >-+
.. - +-|+/
.. ||| |
.. V ---> |V| BCS2 REF
.. SR2 |||
.. -
.. |
..
..
..
..
.. BCS2 BCS1
.. Vout= - Vin SR(+)= ---- SR(-)= ----
.. C C
..
BCS can be the Bartoli Current Source under rheostat control or voltage
control.
F
Fred Bloggs
Fred said:
Maybe a little capacitor multiplication for reasonable CS numbers:
View in a fixed-width font such as Courier.
..
..
.. +----------------[R]-----------------------------+
.. | |
.. | |
.. | |
.. | | |
.. | - |
.. | ||| |
.. | V ---> |V| BCS1 |
.. | SR1 ||| |
.. | - |
.. Vin>--[R]-+--|+\ | C |
.. | >--+--|<|-+-|>|-+-----[R2]----+-----+--||--+->Vout
.. +-|-/ | | | | |
.. | | | [R2/n] | |
.. REF | | | | |
.. +--|>|-+-|<|-+-[Rx]-+-[Rx]-+ +-|-\ |
.. | | | | >-+
.. - +-|-\ | +-|+/
.. ||| | >-+ |
.. V ---> |V| BCS2 +-|+/ REF
.. SR2 ||| |
.. - REF
.. |
..
..
..
..
.. BCS2 BCS1
.. Vout= - Vin SR(+)= ---- SR(-)= ----
.. nC nC
..
F
Fred Bloggs
Fred said:
That's not going to work worth a damn:
View in a fixed-width font such as Courier.
..
..
.. +----------------[R]-----------------------------+
.. | |
.. | |
.. | |
.. | | |
.. | - |
.. | ||| |
.. | V ---> |V| BCS1 |
.. | SR1 ||| |
.. | - |
.. Vin>--[R]-+--|+\ | C |
.. | >--+--|<|-+-|>|-+-----[R2]----------+--||--+->Vout
.. +-|-/ | | | |
.. | | +----[R2/n]---+ | |
.. REF | | | | |
.. +--|>|-+-|<|-+-[Rx]-+-[Rx]-+ +-|-\ |
.. | | | | >-+
.. - +-|-\ | +-|+/
.. ||| | >-+ |
.. V ---> |V| BCS2 +-|+/ REF
.. SR2 ||| |
.. - REF
.. |
..
..
..
..
.. BCS2 BCS1
.. Vout= - Vin SR(+)= ---- SR(-)= ----
.. nC nC
..
F
Fred Bloggs
Winfield said:
If he knows that it's always between 0 and 1V then he can get away with
a digital pot and using fixed current sources and Vin. The digital pot
might be loaded every time he goes to make a transition, and that's
about it:
View in a fixed-width font such as Courier.
..
..
.. +----------------[R]-----------------------------+
.. | |
.. | |
.. | |
.. | | |
.. | - I=W*CS <-> |
.. | ||| +------------+ |
.. | CS1|V| | | |
.. | ||| -- | |
.. | - | / | digi | |
.. Vin>--[R]-+--|+\ | W | \ | pot | C |
.. | >--+--|<|-+-|>|-+--->| / | +--||--+->Vout
.. +-|-/ | | | \ | | |
.. | | | -- | |
.. REF | | | | |
.. +--|>|-+-|<|-+ | +-|-\ |
.. | | | >-+
.. - | +-|+/
.. ||| | |
.. CS2|V| REF REF
.. |||
.. -
.. |
..
..
..
..
.. W*CS2 W*CS1
.. Vout= - Vin SR(+)= ---- SR(-)= ---- 0< W < 1
.. C C
..
W
Winfield Hill
Walter Harley wrote...
.. ,------ R2 --------------------------------,
.. | C |
.. | __ A1 ,---||----+
.. -- R1 --+--|+ \ | __ A2 |
.. | >--+--|>|-- pot --+-- R4--+--|- \ |
.. ,--|-_/ | | | >--+--- out
.. | '--|<|-- pot --+ ,--|+_/
.. gnd | | G = - R2/R1
.. R3 gnd slew set by pots,
.. | R3, R4, C and supply
.. gnd
The slew rate is set independently by the two pots, and by R3,
R4, C and the supply voltages (A1 rails when the output slews).
Low values of R3 will create slow slew rates (note, the two
pots should have series resistors, to avoid zero resistance).
Alternately, here's a circuit that allows for external voltage-
controlled slew rate, but has the same rise and fall rates. It
uses one of NSC's transconductance (current output) amplifiers.
..
.. ,--------- R2 --------,
.. | CA3080 C |
.. in | LM13700 ,---||----+
.. -- R1 --+--|+ \ _ | __ |
.. | (_)--+--|- \ |
.. ,--|-_/ | | >--+--- out
.. | | ,--|+_/ G = -R2/R1
.. gnd | |
.. program max | gnd
.. slew rate |
.. -- R3 --+------ e c -' Vprog
.. | __ b -> I_slew slew = -----
.. '--|- \ | Q1 R3 C
.. | >--' PNP
.. ,--|+_/
.. |
.. gnd
During slewing the LM13700 steers a positive or negative version
of the programmed slewing current into the integrator. This works
well at low slewing currents (even well under 1uA), which you can
get with high values of R3 and/or low programming voltages to R3.
R3 can be made from three resistors in a divider setup.
.. ,------ R2 --------------------------------,
.. | C |
.. | __ A1 ,---||----+
.. -- R1 --+--|+ \ | __ A2 |
.. | >--+--|>|-- pot --+-- R4--+--|- \ |
.. ,--|-_/ | | | >--+--- out
.. | '--|<|-- pot --+ ,--|+_/
.. gnd | | G = - R2/R1
.. R3 gnd slew set by pots,
.. | R3, R4, C and supply
.. gnd
The slew rate is set independently by the two pots, and by R3,
R4, C and the supply voltages (A1 rails when the output slews).
Low values of R3 will create slow slew rates (note, the two
pots should have series resistors, to avoid zero resistance).
Alternately, here's a circuit that allows for external voltage-
controlled slew rate, but has the same rise and fall rates. It
uses one of NSC's transconductance (current output) amplifiers.
..
.. ,--------- R2 --------,
.. | CA3080 C |
.. in | LM13700 ,---||----+
.. -- R1 --+--|+ \ _ | __ |
.. | (_)--+--|- \ |
.. ,--|-_/ | | >--+--- out
.. | | ,--|+_/ G = -R2/R1
.. gnd | |
.. program max | gnd
.. slew rate |
.. -- R3 --+------ e c -' Vprog
.. | __ b -> I_slew slew = -----
.. '--|- \ | Q1 R3 C
.. | >--' PNP
.. ,--|+_/
.. |
.. gnd
During slewing the LM13700 steers a positive or negative version
of the programmed slewing current into the integrator. This works
well at low slewing currents (even well under 1uA), which you can
get with high values of R3 and/or low programming voltages to R3.
R3 can be made from three resistors in a divider setup.
F
Fred Bartoli
Fred Bloggs said:
In that case, if you notice that the A & B points voltages are V(W) +/- one
diode drop, then the current sources can even more be simplified and reduced
to a simple resistor.
I'll also fixe the current divider ratio to say about 50. Then I'll delete
the voltage reference and switch the input to the other opamp input.
Then we arrive at this simple schematics with Vin being directly a 0/5V
logic CMOS output.
+----------------[R]-----------------------------+
| 4.7M |
| ___ |
| .--|___|-+--+15V |
| | A | |
| | | | |
| .-. ---' |
| 100K| | |
| | | |
| '-' 1M |
GND--[R]-+--|+\ | W ___ C |
| >--+--|<|-+-|>|-+------+------|___|-+--||--+->Vout
Vin>-[R]--+-|-/ | A | | | |
| | | .-. | |
[R] | B | | | | |
| +--|>|-+-|<|-+ | |18K +-|-\ |
=== | '-' | >-+
GND .-. | +-|+/
100K| | | |
| | GND GND
'-' 4.7M
| ___
'--|___|-+-- -15V
A |
| |
---'
F
Fred Bloggs
Fred said:
Very good- I added REF in case someone was thinking of single supply. I
prefer more certain protection for the output- not as nice a CMOS input-
but that's just a shift- all Radio Shack stuff, TL082- protection only
kicks in when you have hardware failure of some kind- these values
should get him 0.1->5sec slews, and assumes VCA input is Hi-Z:
View in a fixed-width font such as Courier.
..
..
..
..
.. 0.33U
.. +15V +----+----------||-----+
.. | | | |
.. | --10K | |
.. [1.5M] | / | | |
..Vin>-[10K]-+----|+\ | | \ | | |
.. | | >+-|<|-+--|>|----->| / | | |
.. | +-|-/ | W- | \ | | |
.. | | | --- +--------|-\ |
.. | | | | | | >-----+
.. | | 0.1u| +--------+----|+/ |
.. | +-||--+ | | | |
.. | | | --- | | [1K]
.. | | | | / | | | |
.. | [5k] | W+ | \ | | +-----|>|-----+
.. | | +-|>|-+--|<|----->| / | | | |
.. | | | | \ | | | +-|<|-|<|---+
.. | | [1.5M] --10K | | | [1K]
.. | gnd | | | | +---------+ |
.. | | +----+ | | |
.. | -15V +---|<|-|<|-+ |
.. | | |
.. +----------------[10K]----------------|-------------+-->Vout
.. | |
.. | [1K]
.. | |
.. +-------------+
.. |
.. gnd
..
..
..
..
..
..
..
..
..
..
..
F
Fred Bartoli
Walter Harley said:
Then, to have just one trimpot, you can simply buffer one with another:
1M ( IC(Q1) ~ 10uA )
___
+Vs-----|___|---+-----+------.
| | |
| .-. |
| | |1M |
| | | |
Q1 | '-' |
2N5087 |---+ |
/| | |
| | .-.
| | | |
=== | | | 22M
GND | '-'
2N5087 |----+--->50nA
/|
Q2 |
|
===
GND
but this doubles the trimpot value, so you can do:
1M
___
+Vs-----|___|-+---------+------.
| | |
| | .-.
| | | |
| | | | 10M
| | '-'
| >| |
| 2N5087 |----+--->50nA
| /| B
| Q2 |
\| |
2N5089 |-------+
<| |
| .-.
| | |
=== | |1M
GND '-'
|
===
GND
but it only works for V(B)>0.4V
F
Fred Bloggs
Winfield said:
That's economical but you lose linearity over his 50:1 range with the
pots like that. I hope he uses it though
F
Fred Bartoli
Fred Bloggs said:
Hmmm, it'll work but in case of RadioShack components I guess I'll keep my
fixed 18K-1M current divider and variable current sources:
the 10K trimpots will produce non negligible current offsets due to the
output opamp offset (potentially several mv).
The 1M resistor reduces this current offset to a low enough value that you
won't noptice its effect.
