T
Thomas P. Gootee
Guest
"Best way to get multiple voltage references?"
This may be obvious and trivial for those of you who are "real" analog
designers. But I don't have much experience with voltage references,
and no one else to ask.
The Situation/Background:
I have a self-designed piece of test equipment (a curve tracer), for
which I've almost completed re-designing the power amplifier's
gain-setting network. The amplifier only needs to have six different
0-to-peak output voltages: 0.5, 3, 6, 9, 12, and 15 volts, which are
controlled by a rotary switch.
The old design used an array of resistors, to set the gain for various
combinations of the controls' settings, to maintain the correct p-p
output voltage level. That was somewhat unsatisfactory, for several
reasons.
I now (will) use an AGC (automatic gain control) feedback control
loop, using three opamps, that derives (from the actual output) a DC
voltage equal to the 0-peak voltage and then subtracts a reference
voltage (DC, equal to the *desired* 0-peak amplitude) from it, to get
an "error signal", to drive the LED in a VACTROL (LED and photocell)
used as a variable resistor, to set the LM1875T amplifier's gain. (The
way the circuit is designed, I actually need negative reference
voltages.)
The Quest:
Everything is great, except that I am still trying to decide what is
the "best" way to derive the reference voltages. They wouldn't need to
be able to supply much current, since they will be tied to an opamp's
inverting input, through a 10K resistor. Needless to say, maybe: I
would like them to be as accurate and precise as possible, within my
cost ("low") and size ("small") constraints.
The unit's power supply produces + and - 15 to 18 volts (those CAN be
varied, although they "normally" wouldn't be), and +5 volts, all
usually with <= 10-20 mV of noise.
The available single-sided PCB space for the voltage references is
about 3/8-inch x 3 inches (about 0.95 cm x 7.62 cm), with other traces
right at each boundary. But I could probably squeeze the rest of the
layout to increase that to at least 1/2-inch x 3 1/2 inches (1.27 cm x
8.9 cm). The negative P.S. rail is available at one of the shorter
sides of that area. And ground is available all along one of the long
sides. Five pads, for wires to go to a rotary switch, also need to fit
in there.
I will probably just use the power supply's negative output voltage as
the reference-input for the highest (15 volts 0-peak) output level.
For the five lower voltages, my *first* thought was to just use a
79L12 TO-92 100mA negative 12-volt regulator and four resistive
voltage dividers running from the regulator's output. That would all
fit easily, on the PCB. But that probably wouldn't give precise-enough
reference voltages. I'm thinking that it would be good to have them as
accurate as possible, assuming a reasonably-low cost, but at LEAST
within 1-3 percent, and better if possible. And the 79L12 looks like
it's only good to within +/-5%, not to mention the resistors for the
dividers.
So, I did some research on "voltage references". There sure do seem to
be a lot of different possible ways to do it! I looked at using zener
diodes, opamp and transistor-based methods (with and without zener
diodes), and also those based on actual "precision voltage reference"
products (which seem to also use zener diodes).
I'm currently thinking that I should use something like four TL431CLP
(TO-92) "Adjustable Precision Voltage Reference" components (0.4%, I
think), for the -12, -9v, -6v, and the -3v levels, with a resistive
voltage-divider to derive the -0.5v from the -3v.
Is that the "best" way for me to do it? And, if so, should I choose
something other than a TL431CLP ($0.27 for qty 1 at www.jameco.com)?
And should I just configure each of them independently, or "stack"
them somehow, or what? Are there any decent (or "good enough")
alternatives to using actual "Voltage Reference" components, in this
case?
Assuming the use of "Voltage Reference"s:
Since I've never used "voltage reference" components before, are there
any "gotchas", or any "common practices", etc, that I might not be
aware of? Any good appnotes out there, for them? (I can't seem to
find very much. Maybe they're too trivial to use, so appnotes aren't
really needed?)
Also, for the -0.5v reference, is a resistive voltage-divider from the
-3v the best way to go? If so, it sure would be nice to get away with
using 1% resistors ($0.02 ea for qty 200, at www.mouser.com), unless
someone knows how I can get higher-precision resistors for less than
$1 or $2 each (although I will certainly pay for the 0.1% resistors,
if I have to, in order to almost match the accuracy and precision of
the other reference voltages).
About the resistive voltage-divider to get the -0.5v ref from the -3v
ref: I read a newsgroup post that suggested using several resistors in
series, for the top of the divider, and several in parallel, for the
bottom of the divider. I think that they also mentioned that that is
how it's done on ICs. But, if so, what is the rationale for that?
(Higher precision? Redundancy? Lower noise? Or what?) I may be a
little short of PCB space, too. But if there's a good-enough reason
for doing it that way, I can try, and can most-likely fit it in.
(What?! Even MORE of those expensive 0.1% resistors?!)
Sorry that this is so lengthy, and for asking so many questions at
once. ANY help, advice, or ideas will be *GREATLY* appreciated.
Regards,
Tom Gootee
tomg AT fullnet.com
http://www.fullnet.com/u/tomg
P.S. There are photos of the curve tracer unit, and photos of the
prototype's screen-displays, etc, at
http://www.fullnet.com/u/tomg/gooteect.htm . (It's pretty neat!)
This may be obvious and trivial for those of you who are "real" analog
designers. But I don't have much experience with voltage references,
and no one else to ask.
The Situation/Background:
I have a self-designed piece of test equipment (a curve tracer), for
which I've almost completed re-designing the power amplifier's
gain-setting network. The amplifier only needs to have six different
0-to-peak output voltages: 0.5, 3, 6, 9, 12, and 15 volts, which are
controlled by a rotary switch.
The old design used an array of resistors, to set the gain for various
combinations of the controls' settings, to maintain the correct p-p
output voltage level. That was somewhat unsatisfactory, for several
reasons.
I now (will) use an AGC (automatic gain control) feedback control
loop, using three opamps, that derives (from the actual output) a DC
voltage equal to the 0-peak voltage and then subtracts a reference
voltage (DC, equal to the *desired* 0-peak amplitude) from it, to get
an "error signal", to drive the LED in a VACTROL (LED and photocell)
used as a variable resistor, to set the LM1875T amplifier's gain. (The
way the circuit is designed, I actually need negative reference
voltages.)
The Quest:
Everything is great, except that I am still trying to decide what is
the "best" way to derive the reference voltages. They wouldn't need to
be able to supply much current, since they will be tied to an opamp's
inverting input, through a 10K resistor. Needless to say, maybe: I
would like them to be as accurate and precise as possible, within my
cost ("low") and size ("small") constraints.
The unit's power supply produces + and - 15 to 18 volts (those CAN be
varied, although they "normally" wouldn't be), and +5 volts, all
usually with <= 10-20 mV of noise.
The available single-sided PCB space for the voltage references is
about 3/8-inch x 3 inches (about 0.95 cm x 7.62 cm), with other traces
right at each boundary. But I could probably squeeze the rest of the
layout to increase that to at least 1/2-inch x 3 1/2 inches (1.27 cm x
8.9 cm). The negative P.S. rail is available at one of the shorter
sides of that area. And ground is available all along one of the long
sides. Five pads, for wires to go to a rotary switch, also need to fit
in there.
I will probably just use the power supply's negative output voltage as
the reference-input for the highest (15 volts 0-peak) output level.
For the five lower voltages, my *first* thought was to just use a
79L12 TO-92 100mA negative 12-volt regulator and four resistive
voltage dividers running from the regulator's output. That would all
fit easily, on the PCB. But that probably wouldn't give precise-enough
reference voltages. I'm thinking that it would be good to have them as
accurate as possible, assuming a reasonably-low cost, but at LEAST
within 1-3 percent, and better if possible. And the 79L12 looks like
it's only good to within +/-5%, not to mention the resistors for the
dividers.
So, I did some research on "voltage references". There sure do seem to
be a lot of different possible ways to do it! I looked at using zener
diodes, opamp and transistor-based methods (with and without zener
diodes), and also those based on actual "precision voltage reference"
products (which seem to also use zener diodes).
I'm currently thinking that I should use something like four TL431CLP
(TO-92) "Adjustable Precision Voltage Reference" components (0.4%, I
think), for the -12, -9v, -6v, and the -3v levels, with a resistive
voltage-divider to derive the -0.5v from the -3v.
Is that the "best" way for me to do it? And, if so, should I choose
something other than a TL431CLP ($0.27 for qty 1 at www.jameco.com)?
And should I just configure each of them independently, or "stack"
them somehow, or what? Are there any decent (or "good enough")
alternatives to using actual "Voltage Reference" components, in this
case?
Assuming the use of "Voltage Reference"s:
Since I've never used "voltage reference" components before, are there
any "gotchas", or any "common practices", etc, that I might not be
aware of? Any good appnotes out there, for them? (I can't seem to
find very much. Maybe they're too trivial to use, so appnotes aren't
really needed?)
Also, for the -0.5v reference, is a resistive voltage-divider from the
-3v the best way to go? If so, it sure would be nice to get away with
using 1% resistors ($0.02 ea for qty 200, at www.mouser.com), unless
someone knows how I can get higher-precision resistors for less than
$1 or $2 each (although I will certainly pay for the 0.1% resistors,
if I have to, in order to almost match the accuracy and precision of
the other reference voltages).
About the resistive voltage-divider to get the -0.5v ref from the -3v
ref: I read a newsgroup post that suggested using several resistors in
series, for the top of the divider, and several in parallel, for the
bottom of the divider. I think that they also mentioned that that is
how it's done on ICs. But, if so, what is the rationale for that?
(Higher precision? Redundancy? Lower noise? Or what?) I may be a
little short of PCB space, too. But if there's a good-enough reason
for doing it that way, I can try, and can most-likely fit it in.
(What?! Even MORE of those expensive 0.1% resistors?!)
Sorry that this is so lengthy, and for asking so many questions at
once. ANY help, advice, or ideas will be *GREATLY* appreciated.
Regards,
Tom Gootee
tomg AT fullnet.com
http://www.fullnet.com/u/tomg
P.S. There are photos of the curve tracer unit, and photos of the
prototype's screen-displays, etc, at
http://www.fullnet.com/u/tomg/gooteect.htm . (It's pretty neat!)