Well I knew you'd say that, of course, what with the CYA in your line of work. I admit to using JFET-input op-amps a tad 'over-the-top' way back when, maybe even the TL082. Or maybe the LF442.
Not for serious work, but, with due checking, I wouldn't rule that out either.
What about anti-parallel zeners across the feedback resistor to kill the gain. As long the output doesn't bury itself into the rails, there is 0V across the inputs of the op-amp. Plenty of op- amps will take Vcm to the rails. In this case, Vcm is really somewhat less than the rails (16.6V) because of the divider on the non-inverting input.
Actually I don't think it's a real problem, despite the abs. max. rating.
If you look at the schematic in Figure 2 (such as it is), there are effectively back-to-back diodes across the inputs (the E-B junctions of Q5 and Q6, with series resistors on each input (of unstated value) internal to the chip).
So it should work reliably without any added stuff, IMHO, provided the input current is limited to a couple of mA.
As a corollary, the LM6132 would make a lousy comparator.
Else I could lose my shirt, literally. You can contractually exclude the consequences of honest mistakes and I always do (or decline the request) but the millisecond something can be construed as grossly negligent for a person skilled in the art you are doomed.
On the lab bench or for hobby there is (usually) nothing wrong with that.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Yes, I did read the entire solution. Your "solution" was to reduce the gain of the diff amp to about 3 and pick up the remaining gain by increasing the value of the sense resistor(s). To get back to a gain of 30 would require increasing the value of the sense resistors by a factor of ten, changing the drop at 1 amp to about 2.5 volts. Since I started off with a rather marginal 2 volts of headroom anyway (17 volts raw supply for a 15 volt maximum output), that would mean that the raw supply would be somewhere around 14.5 for a 15 volt output. Not real easy to do, is it?
I don't hardly think that's "huffy", do you?
Jim
-- "It is the mark of an educated mind to be able to entertain a thought without accepting it." --Aristotle
This answer didn't seem huffy, but your first one did. I apologize if you didn't intend it that way.
Anyway, the Linear Tech LT1366CS8 part looks like it's a drop in for your LM358 SO part. It has rail-to-rail input common mode range and 36V max supply. It also drives much closer to the negative supply (GND) than the LM358, so when your supply is putting out no current then your voltmeter will read properly.
Bob
--
== All google group posts are automatically deleted due to spam ==
Can you put a voltage divider on the output of the sense resistor with manhattan construction, and increase gain?
I'm also wondering about the LM317 dropout at 15V, as the current rises toward 1 amp. Maybe you could make it into a lesson - watch the regulator fail to maintain 15V as the load current increases.
I take "magnitude" to mean "absolute value", i.e. a) Vin must be between the supply rails, and b) |Vin| < 15v.
It's a question of the reverse breakdown voltage of the input FETs' gates, and that voltage here is guaranteed >= 30v.
That limit is usually to protect bipolar input stages from reverse breakdown, emitter-to-base. That limits most bipolar input stages to a few volts differential, max. Precision parts often have clamps to protect the input transistors from beta degradation resulting from e-b breakdown.
JFET input op amps aren't usually so constrained, ergo the
This approach reduces the CMRR error, but still requires the op-amp handle inputs to +Vcc, and have an output that swings to [Vcc-Vbe(Q1)] (a pull-up to Vcc fixes that). Output to "meter" is hi-z:
====== FIG. 4 ====== If Jim can just hack a few traces and move the sense resistor, this circuit uses the fewest, cheapest parts, and has the best accuracy:
The LM358 inputs are rated to operate correctly down to GND, making this version especially easy. 2.7k in series with the "+" input would be a good idea, just in case.
On Sat, 22 Nov 2008 17:24:48 GMT, James Arthur wrote: [snip]
Sure there is. When specs are so loose that they aren't even measured during manufacture.
Really? Looks to me like they're JFET's, are they not?
[snip]
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
formatting link
| 1962 | I love to cook with wine Sometimes I even put it in the food
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
Not quite: If there is a short the full input voltage will show up for a very brief moment. I don't know his circuit but if the opamp then fails even just functionally by flipping its polarity or whatever ... *WHADDABAM*
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
If somebody has an older TL071/TL072 spec sheet, maybe they could check to see if this spec has changed with the 2005 version (Either ST or TI)... Seems I remember it USED to be that the input could exceed the V+ line by a couple hundred millivolts. It has been used it this way fo high side current sensors before. Maybe they have decided this is not such a good idea anymore ??
My "revised 1983" data book says the same thing, but I'd think you can probably get away with as much as 0.3V above supply.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.