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
