adding an enable to a bipolar op-amp, how???

Can you enable Vref? If it's zero at power-up, so is the OpAmp output.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

You could use a tiny SPDT analog switch to open the 4-volt path, or even switch it to ground. Like an Sn74LVC1G66 or something.

Or use an opamp that has a powerdown input.

John

It probably draws so little current that you could power it via a

74HC04 output. That way VDD will be forced to zero when off, rather than floating.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

There are some amplifiers around with clamping terminals. The Comlinear CLC502

comes to mind, but National Semiconductor has discontinued it.

The compensation terminal on the LM301 and the (rather nasty) uA748 can be used as a clamping input - in addition to the 30pF to ground compensation capacitor (built into modern op amps) you can hook up a pair of small diodes (!N918 or similar) that draw current if the compensation point goes above some positive voltage, or sink current if the output goes below some negative voltage.

I last tried this around 1975, so the details are a little vague in my mind.

-------------- Bill Sloman, Nijmegen

Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V using an op-amp as a differential amplifier. On powerup, the DAC is reset to 0V, which obviously makes the output of the op-amp -2.048V. I want the output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing? All of my solutions seem too complicated for such a seemingly simple requirement. :-(

Thanks,

Hank

I thought all you guys draw in ASCII :-)

Ok, I did a quick drawing and posted to alt.binaries.schematics.electronic under the title "Bipolar output from single supply DAC"

Thanks,

Hank

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

mmm, it is a three terminal part (+5V, Gnd, and 4.096V output) but I suppose I could put a smallish logic level P-channel fet between its VCC terminal and +5V...

that seems easy enough, huh?

Here I was trying to muck with the final output...

guess that is why I pay you the big bucks...oh wait... :-)

Thanks,

Hank

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Actually, upon further thought, I am going to switch to a shunt type Vref. The part I was going to use, AD158x has an inrush current of 15mA or more. It has a low overhead requirement of only 200mV, but if the pfet I choose has a typical Rdson of ~100 ohms, that is 1.5V dropped across the fet...oops!

I think a typical resistor used with a shunt is about 1k, meaning only 1mA through the pfet, or a drop of 100mV. The Vref input to the DAC has a 160k impedance, the input impedance of my op-amp configuration is going to be equal to R3, so probably about 20k...I guess I should buffer the output from the shunt?

I guess there never is a really simple solution...

Use an analog switch on the op-amp output ?

Graham

Maybe think about switching the bias on the differential amplifier.

Option1. ~~~~~~~~ Switch 2R R 4.096Vref -+----+/+------/\\/\\---+---/\\/\\--+--> Vout | _ | +--|- \\ | |Op >--+ +--|+_/ | DAC(at 0V) +------------/\\/\\----+---/\\/\\--+0V 2R R

At power-up Switch= OFF, so there is no bias on the diff-amp. Needs a good switch because the switch-resistance is in circuit during normal operation.

Option2. ~~~~~~~~

2R R 4.096Vref -+-------------/\\/\\---+---/\\/\\--+--> Vout | | _ | | +--|- \\ | | Switch 4R/3 |Op >--+ +---+/+------/\\/\\----+--|+_/ | DAC(at 0V) +------------/\\/\\----+---/\\/\\--+0V 2R R

At switch-on DAC=0V, Switch= ON, and the 4R/3 is in circuit. This moves the diff-amp into equal 2R/R on both legs, with both inputs the same. Vout= 0. Switch= OFF for normal operation.

--
Tony Williams.

How about this one?

R3 R4 ___ ___ o-|___|-------+-----------------|___|-----. +4.096V | | | | .------------. ' Enable | | +------|----------------+ o-----------| | | | | | | 'o | |\\ | +/-2.048V out | | __--o-|------|-\\ ' | '----+ | | >------+----o R1 | | .---|+/ ___ +---|------o | | |/ o-|___|-+ | __--o-|--+ +---|------o | DAC 0-4V | '------------' .-. 2/3 4053 | | R2 | | '-' |

GND

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

This is probably a bit better in general, and perhaps better for this application- it outputs zero rather than Vdac*(2/3) when 'disabled' and still has cancellation of Ib* switch resistance and switch resistance variations over input voltage.

R3 R4 ___ ___ o-|___|-------.-----------------|___|-----. +4.096V | | | | .------------. ' | | +------|----------------+ | | | | Iba | | | 'o | ------+----o | | .---|+/ GND-|------o | | |/ ___ | __--o-|--+ o-|___|---+-|-- ---o |

It would be simpler to keep the Enable control out of the precision signal paths which would mean something like controlling the V- to the OA, a RRIO type will then simply rail out at 0V. A Tiny Logic switch will do. View in a fixed-width font such as Courier.

. . +5 . | . | . 4.096V >--[R3]-+-[R4]------|----. . | | | . | | OA | . '---------|-\\ | . | >---+-> Out . .---------|+/ . | | . Vin >--[R1]-+-[R2]-. | . | | . | | | | | | | | | | . +5V .----+ | . | | | | . | GND | | . __ |- | | . EN >--o| | | . |- | | . | | | . [1M] | | . | | \\ | . +---------| o---' . | | / . [1M] | GATE . | | . '-----------+ . | . -5V . . . .

[snip]

A caveat Fred. A 'RRIO' opamp whose o/p stage is a bipolar transistor with a parallel c-c FET for the final pulldown will get it's Vout lifted by the current through R3+R4. The '324-type is quite an ambush for this effect when running on single rail. Must use an opamp with a genuine CMOS o/p stage.

--
Tony Williams.

On 07/12/2005 the venerable Hank etched in runes:

Use a CA3130 op-amp. It has an output enable pin.

--
John B

I noticed, but didn't think it needed comment ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

R 2R ~ ~~ Quietly correcting a drawing typo before anyone else notices and comments.

--
Tony Williams.

Right- well anyone who would use an LM324 for a 12-bit system would deserve it- maybe something like even a LT1013 would be good- and this will sink the feedforward bias. Then U04 type for the pulldown gate on V- with a Miller cap to make the transition without too much disruption.