Looking for a circuit to convert DAC output to +/-5v range..

[Kasterborus]

Hi,

I recently got the TLV5618 DAC working with an Ardiuno board. So I can
now output signals in the 0 to 5v range. I would like to use these
signals to control a laser galvanometer that requires -5 to +5v signal
range.

I've been trying to scale and translate the signal using a TL082 op-
amp, but not been successful so far - can anyone show me how I can
convert the signal into the required output.

Datasheet for the TLV5618 is here:
http://focus.ti.com/lit/ds/symlink/tlv5618a.pdf

Any help would be greatly appreciated.

Dave

 

[Chris]

On Nov 18, 10:07 pm, Kasterborus <kasterbo...@yahoo.com> wrote:

Hi,

I recently got the TLV5618 DAC working with an Ardiuno board. So I can
now output signals in the 0 to 5v range. I would like to use these
signals to control a laser galvanometer that requires -5 to +5v signal
range.

I've been trying to scale and translate the signal using a TL082 op-
amp, but not been successful so far - can anyone show me how I can
convert the signal into the required output.

Datasheet for the TLV5618 is here:http://focus.ti.com/lit/ds/symlink/tlv5618a.pdf

Any help would be greatly appreciated.

Dave

Hi, Dave. You need an op amp configured as a difference amplifier
following the DAC output.

Another way of expressing your problem is to say you want to subtract
2.5V from your output, making your 0V to +5V signal into a -2.5V to
+2.5V signal, and then have a gain of 2, making it -5V to +5V.

Look at the difference amp on the bottom of the first page of National
Semiconductor's AN-31, "Op Amp Circuit Collection":

http://www.national.com/an/AN/AN-31.pdf#page=1

Assuming you have a +/-5V supply available, use a rail-to-rail input
and output op amp, apply your DAC output at V2 and a +2.5V reference
at V1, and use 200K resistors for R2 and R4, and 100K for R1 and R3.
Your programmed output will be:

Vout = 2 * (DAC output - 2.5V)

which should do the job.

I'm guessing you're just using the +5V supply as REF (pin 6 on the 8-
pin DIP pkg). If so, you might want to use a dual op amp, and just
use a voltage divider to get the 2.5V. You can then use the second op
amp as a voltage follower to apply the +2.5V to the difference amp.
And if the zero point of your galvanometer is important, you could use
a 10-turn pot to tweak in a fairly exact zero point.

The whole circuit would look something like this (view in fixed font
or M$ Notepad):

|
| VCC
| +
| |
| .-.
| 10K| | 200K
| | | ___
| '-' .---|___|---.
| | | VCC |
| .-. |\ | + |
| 1K| |<---|+\ ___ | |\| |
| | | | >---o-|___|-o--|-\ |
| '-' .--|-/ | 100K | >-----'
| | | |/| | .--|+/
| .-. | - | | |/
| 10K| | | VEE | |
| | | '---------' | ___
| '-' o---|___|---.
| | | 200K |
| === .-. ===
| GND 100K| | GND
| | |
| '-'
| DAC Output |
| o-------'
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

This should work pretty well if you use 1% resistors and a 10-turn
tweaker pot.

Good luck
Chris

 

[Kasterborus]

Hi Chris,

Thanks so much for putting that together - that's exactly what I was
trying to do, but my implementation skills still need some work. I
think I have the parts for this in my "junk" draw - so I'll give it a
try.

One question - I've been using power from the galvo amp PSU which is
+15v / -15v so this would be my Vcc/Vee respectively? Do I need this
much, or would a +6/-6v power supply be better?

Dave

 

[Chris]

On Nov 19, 9:14 am, Kasterborus <kasterbo...@yahoo.com> wrote:

Hi Chris,

Thanks so much for putting that together - that's exactly what I was
trying to do, but my implementation skills still need some work. I
think I have the parts for this in my "junk" draw - so I'll give it a
try.

One question - I've been using power from the galvo amp PSU which is
+15v / -15v so this would be my Vcc/Vee respectively? Do I need this
much, or would a +6/-6v power supply be better?

Dave

Hi, Dave. I'm sure you're not using the +15V for the DAC (which has a
maximum power supply voltage of 7VDC). But the op amp setup will work
better at +/-15V, because you won't need a rail-to-rail op amp -- just
about any low drift general purpose op amp will do fine. Even an old
fashioned LM741 will work pretty well, only drifting ten millivolts or
so with moderate changes in temp.

Good luck
Chris