TL081 on Gnd/+9V, PIC on Gnd/+5V ?

Hello,
I've looked all the datasheets of the TL081/TL082, and
Googled around with keywords like "single supply TL081"
and others, but didn't find a schematic or an answer to
my question, which in its substance is:

Having gnd and +5V which feed my PIC, if I add a +9V
line, can I place a TL081 between gnd and +9V (it would
be out of spec of 1V, but it's not the problem I'm
asking about) and expect it to work well? I'm using
it as a simple voltage follower. Do I need to make a
virtual ground (e.g. via two 470 ohm resistors) and
make some current flow between it and the TL081 output?
(from the datasheet the TL081 doesn't seem to require
any current flowing between its output and ground, but
there's no mention to a circuit like this anyway).
The opamp + input would receive voltages between +2V
and +5V, i.e. (relative to the midpoint of its gnd/9V
power) -2.5V to +0.5V. I need to attach its output to
the PIC's ADC, where I'll read from 2V to 5V. Of course
I can't use a TL081 on the gnd/+5V power bus, and a
dual -5V/gnd/+5V would force me to modify my already
built PSU and, more than that, wouldn't give me the
2V..5V range into the PIC ADC that I need, since the
TL081 is not rail-to-rail. Yes I know there are nice
rail-to-rail opamps, even FET-input ones, but I've a
lot of TL081 and if the circuit above has no problems
working, why not use it? It would take me many days
to order another opamp, I'd have to throw the current
prototype to make another one from scratch, etc..

The circuit I'm thinking of is this:

Gnd
|
\
/ 470 +9V +5V
\ | |
| 470 | |
--------------*---/\/\/----* |
- | |
input | |
+ | |
---------------------- + | |
TL081 --*--------- ADC PIC
.-- - | | |
| | | |
| Gnd | Gnd
| |
'-------------'

input ranges from -0.5V to +2.5V, I expect to get on
the PIC ADC +5V to +2V in response to this input range.

Greets and Thanks a lot in advance for your help!
TPM

 

[John Popelish]

tpm@againstallspam.com wrote:

Hello,
I've looked all the datasheets of the TL081/TL082, and
Googled around with keywords like "single supply TL081"
and others, but didn't find a schematic or an answer to
my question, which in its substance is:

Having gnd and +5V which feed my PIC, if I add a +9V
line, can I place a TL081 between gnd and +9V (it would
be out of spec of 1V, but it's not the problem I'm
asking about) and expect it to work well?

But it is the problem. The opamp has an input common mode range that
is guaranteed to be no better than +-11 volts when the chip is powered
from +-15 volts. It may be better or worse than that with a +9 and
zero volt supply, but even if it has the same overhead requirements,
that means that the inputs may operate correctly only between 4 and 5
volts.

I'm using
it as a simple voltage follower. Do I need to make a
virtual ground (e.g. via two 470 ohm resistors) and
make some current flow between it and the TL081 output?
(from the datasheet the TL081 doesn't seem to require
any current flowing between its output and ground, but
there's no mention to a circuit like this anyway).
The opamp + input would receive voltages between +2V
and +5V, i.e. (relative to the midpoint of its gnd/9V
power) -2.5V to +0.5V. I need to attach its output to
the PIC's ADC, where I'll read from 2V to 5V.

If you bias the input voltage to swing around +4.5 volts, then that is
also the range the output voltage will swing around. Followers just
follow the input voltage.

Of course
I can't use a TL081 on the gnd/+5V power bus, and a
dual -5V/gnd/+5V would force me to modify my already
built PSU and, more than that, wouldn't give me the
2V..5V range into the PIC ADC that I need, since the
TL081 is not rail-to-rail.

It won't even work very well with a +- 5 volt supply. That would
provide an input range between +-1 volt.

Yes I know there are nice
rail-to-rail opamps, even FET-input ones, but I've a
lot of TL081 and if the circuit above has no problems
working, why not use it?

Because it was designed to operate between +- 15 volt rails.

It would take me many days
to order another opamp, I'd have to throw the current
prototype to make another one from scratch, etc..

You will waste more time trying to get this to work as you want.

The circuit I'm thinking of is this:

Gnd
|
\
/ 470 +9V +5V
\ | |
| 470 | |
--------------*---/\/\/----* |
- | |
input | |
+ | |
---------------------- + | |
TL081 --*--------- ADC PIC
.-- - | | |
| | | |
| Gnd | Gnd
| |
'-------------'

input ranges from -0.5V to +2.5V, I expect to get on
the PIC ADC +5V to +2V in response to this input range.

Greets and Thanks a lot in advance for your help!
TPM

--
John Popelish

 

[Andrew Holme]

tpm@againstallspam.com wrote:

Hello,
I've looked all the datasheets of the TL081/TL082, and
Googled around with keywords like "single supply TL081"
and others, but didn't find a schematic or an answer to
my question, which in its substance is:

Having gnd and +5V which feed my PIC, if I add a +9V
line, can I place a TL081 between gnd and +9V (it would
be out of spec of 1V, but it's not the problem I'm
asking about) and expect it to work well? I'm using
it as a simple voltage follower. Do I need to make a
virtual ground (e.g. via two 470 ohm resistors) and
make some current flow between it and the TL081 output?
(from the datasheet the TL081 doesn't seem to require
any current flowing between its output and ground, but
there's no mention to a circuit like this anyway).
The opamp + input would receive voltages between +2V
and +5V, i.e. (relative to the midpoint of its gnd/9V
power) -2.5V to +0.5V. I need to attach its output to
the PIC's ADC, where I'll read from 2V to 5V. Of course
I can't use a TL081 on the gnd/+5V power bus, and a
dual -5V/gnd/+5V would force me to modify my already
built PSU and, more than that, wouldn't give me the
2V..5V range into the PIC ADC that I need, since the
TL081 is not rail-to-rail. Yes I know there are nice
rail-to-rail opamps, even FET-input ones, but I've a
lot of TL081 and if the circuit above has no problems
working, why not use it? It would take me many days
to order another opamp, I'd have to throw the current
prototype to make another one from scratch, etc..

The circuit I'm thinking of is this:

Gnd
|
\
/ 470 +9V +5V
\ | |
| 470 | |
--------------*---/\/\/----* |
- | |
input | |
+ | |
---------------------- + | |
TL081 --*--------- ADC PIC
.-- - | | |
| | | |
| Gnd | Gnd
| |
'-------------'

input ranges from -0.5V to +2.5V, I expect to get on
the PIC ADC +5V to +2V in response to this input range.

Greets and Thanks a lot in advance for your help!
TPM

Have you drawn the input polarity wrong, or do you mean +0.5 to -2.5V?

4.5 + 0.5 = 5.0
4.5 - 2.5 = 2.0

Looks like you've got your signs wrong somewhere. Apart from that: yes - it
should work; but why do you need the op-amp at all? Is the input impedance
of the PIC too low?

 

[Robert Monsen]

tpm@againstallspam.com wrote:

Hello,
I've looked all the datasheets of the TL081/TL082, and
Googled around with keywords like "single supply TL081"
and others, but didn't find a schematic or an answer to
my question, which in its substance is:

Having gnd and +5V which feed my PIC, if I add a +9V
line, can I place a TL081 between gnd and +9V (it would
be out of spec of 1V, but it's not the problem I'm
asking about) and expect it to work well? I'm using
it as a simple voltage follower. Do I need to make a
virtual ground (e.g. via two 470 ohm resistors) and
make some current flow between it and the TL081 output?
(from the datasheet the TL081 doesn't seem to require
any current flowing between its output and ground, but
there's no mention to a circuit like this anyway).
The opamp + input would receive voltages between +2V
and +5V, i.e. (relative to the midpoint of its gnd/9V
power) -2.5V to +0.5V. I need to attach its output to
the PIC's ADC, where I'll read from 2V to 5V. Of course
I can't use a TL081 on the gnd/+5V power bus, and a
dual -5V/gnd/+5V would force me to modify my already
built PSU and, more than that, wouldn't give me the
2V..5V range into the PIC ADC that I need, since the
TL081 is not rail-to-rail. Yes I know there are nice
rail-to-rail opamps, even FET-input ones, but I've a
lot of TL081 and if the circuit above has no problems
working, why not use it? It would take me many days
to order another opamp, I'd have to throw the current
prototype to make another one from scratch, etc..

A follower for an opamp is just the V- input connected to the output.
The output will follow the V+ input then. You don't need a voltage
divider, which will cause problems with power supply noise rejection anyway.

If you need to map the input voltage down to something in the 5V range,
use a voltage divider on the input to ground.

Do any voltage mapping and inversion in software.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[dB]

tpm@againstallspam.com wrote in message news:<YwpPd.532361$b5.24786094@news3.tin.it>...

Hello,
I've looked all the datasheets of the TL081/TL082, and
Googled around with keywords like "single supply TL081"
and others, but didn't find a schematic or an answer to
my question, which in its substance is:

Do I need to make a virtual ground ... ?

The opamp + input would receive voltages between +2V
and +5V.

No. Just connect the input voltage to the non-inverting input. Like this:




+9 +5
| |
| |
| |
| |
------------------- + | |
+ TL081 --.--------- ADC PIC
i .-- - | | |
n | | | |
p | Gnd | Gnd
u | |
t '-------------'
-
-------------------------------------------------Gnd

 

[Jamie]

tpm@againstallspam.com wrote:

Hello,
I've looked all the datasheets of the TL081/TL082, and
Googled around with keywords like "single supply TL081"
and others, but didn't find a schematic or an answer to
my question, which in its substance is:

Having gnd and +5V which feed my PIC, if I add a +9V
line, can I place a TL081 between gnd and +9V (it would
be out of spec of 1V, but it's not the problem I'm
asking about) and expect it to work well? I'm using
it as a simple voltage follower. Do I need to make a
virtual ground (e.g. via two 470 ohm resistors) and
make some current flow between it and the TL081 output?
(from the datasheet the TL081 doesn't seem to require
any current flowing between its output and ground, but
there's no mention to a circuit like this anyway).
The opamp + input would receive voltages between +2V
and +5V, i.e. (relative to the midpoint of its gnd/9V
power) -2.5V to +0.5V. I need to attach its output to
the PIC's ADC, where I'll read from 2V to 5V. Of course
I can't use a TL081 on the gnd/+5V power bus, and a
dual -5V/gnd/+5V would force me to modify my already
built PSU and, more than that, wouldn't give me the
2V..5V range into the PIC ADC that I need, since the
TL081 is not rail-to-rail. Yes I know there are nice
rail-to-rail opamps, even FET-input ones, but I've a
lot of TL081 and if the circuit above has no problems
working, why not use it? It would take me many days
to order another opamp, I'd have to throw the current
prototype to make another one from scratch, etc..

The circuit I'm thinking of is this:

Gnd
|
\
/ 470 +9V +5V
\ | |
| 470 | |
--------------*---/\/\/----* |
- | |
input | |
+ | |
---------------------- + | |
TL081 --*--------- ADC PIC
.-- - | | |
| | | |
| Gnd | Gnd
| |
'-------------'

input ranges from -0.5V to +2.5V, I expect to get on
the PIC ADC +5V to +2V in response to this input range.

Greets and Thanks a lot in advance for your help!
TPM

just supply the TL081 with 6 or more volts.

if your worried about the output exceeding at any
given point, then put a 5.1 zener diode to ground
on the TL081 decoupled with a R.
using the TL081 for a follower, output to - input
(loop back), + is the input.

 

[peterken]

<tpm@againstallspam.com> wrote in message
news:YwpPd.532361$b5.24786094@news3.tin.it...

Hello,
I've looked all the datasheets of the TL081/TL082, and
Googled around with keywords like "single supply TL081"
and others, but didn't find a schematic or an answer to
my question, which in its substance is:

Having gnd and +5V which feed my PIC, if I add a +9V
line, can I place a TL081 between gnd and +9V (it would
be out of spec of 1V, but it's not the problem I'm
asking about) and expect it to work well? I'm using
it as a simple voltage follower. Do I need to make a
virtual ground (e.g. via two 470 ohm resistors) and
make some current flow between it and the TL081 output?
(from the datasheet the TL081 doesn't seem to require
any current flowing between its output and ground, but
there's no mention to a circuit like this anyway).
The opamp + input would receive voltages between +2V
and +5V, i.e. (relative to the midpoint of its gnd/9V
power) -2.5V to +0.5V. I need to attach its output to
the PIC's ADC, where I'll read from 2V to 5V. Of course
I can't use a TL081 on the gnd/+5V power bus, and a
dual -5V/gnd/+5V would force me to modify my already
built PSU and, more than that, wouldn't give me the
2V..5V range into the PIC ADC that I need, since the
TL081 is not rail-to-rail. Yes I know there are nice
rail-to-rail opamps, even FET-input ones, but I've a
lot of TL081 and if the circuit above has no problems
working, why not use it? It would take me many days
to order another opamp, I'd have to throw the current
prototype to make another one from scratch, etc..

The circuit I'm thinking of is this:

Gnd
|
\
/ 470 +9V +5V
\ | |
| 470 | |
--------------*---/\/\/----* |
- | |
input | |
+ | |
---------------------- + | |
TL081 --*--------- ADC PIC
.-- - | | |
| | | |
| Gnd | Gnd
| |
'-------------'

input ranges from -0.5V to +2.5V, I expect to get on
the PIC ADC +5V to +2V in response to this input range.

Greets and Thanks a lot in advance for your help!
TPM

better try using a rail-to-rail opamp working at 5V, might be lots simpler.
examples:
max 4041
lmc 6464 (quad)
el 5027
or just try googling for "rail to rail" (with quotes)