| Author |
Message |
Guest
|
 Posted:
Sat Feb 12, 2005 9:46 pm Post subject:
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
|
|
| Back to top |
|
 |
Ken Smith
Guest
|
| Posted:
Sat Feb 12, 2005 9:46 pm Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
In article <420E27D2.70709@nospam.com>, Fred Bloggs <nospam@nospam.com> wrote:
| Quote: | This should just make it:
View in a fixed-width font such as Courier.
+9V +5V
| |
(+) | 10K
Vin-------|\ 1n4148 |
| >--|<|-----+---> TO PIC A/D
2V<Vin<5V ,---|/ |
| (-) | |
to gnd '-----|----------'
|
------------+--------------
common gnd
|
I don't think the TL081 will swing within 2 diodes of its (-) rail. The
1N4148 makes it 3 diodes so I don't think the 2V end of the swing will
work.
You could use a Schottky.
I don't know enough about a PIC to be able to say for sure but on many
ADCs a high frequency high impedance input is a very bad idea.
It may be simpler to get a better op-amp but an RC between the op-amp and
the ADC may be needed.
--
--
kensmith@rahul.net forging knowledge |
|
| Back to top |
|
|
Fred Bloggs
Guest
|
| Posted:
Sat Feb 12, 2005 9:59 pm Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
| Quote: | 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.
|
This should just make it:
View in a fixed-width font such as Courier.
+9V +5V
| |
(+) | 10K
Vin-------|\ 1n4148 |
| >--|<|-----+---> TO PIC A/D
2V<Vin<5V ,---|/ |
| (-) | |
to gnd '-----|----------'
|
------------+--------------
common gnd
|
|
| Back to top |
|
|
Fred Bloggs
Guest
|
| Posted:
Sat Feb 12, 2005 11:24 pm Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
Ken Smith wrote:
| Quote: | In article <420E27D2.70709@nospam.com>, Fred Bloggs <nospam@nospam.com> wrote:
This should just make it:
View in a fixed-width font such as Courier.
+9V +5V
| |
(+) | 10K
Vin-------|\ 1n4148 |
| >--|<|-----+---> TO PIC A/D
2V<Vin<5V ,---|/ |
| (-) | |
to gnd '-----|----------'
|
------------+--------------
common gnd
I don't think the TL081 will swing within 2 diodes of its (-) rail. The
1N4148 makes it 3 diodes so I don't think the 2V end of the swing will
work.
|
The OA need only achieve one diode drop below 2V or 1.3V to pull the
output to 2V. The "typical" characteristics show the OA achieving rails
within 0.625V with 10K load to GND and +/-5V supplies. Maybe he should
drop a 10K from OA output to GND to improve things:
View in a fixed-width font such as Courier.
+9V +5V
| |
(+) | 10K
Vin-------|\ 1n4148|
| >--+--|<|--+---> TO PIC A/D
2V<Vin<5V ,---|/ | |
to gnd | (-) | | |
'-----|----------'
| |
| 10K
| |
------------+--+-----------
common gnd
| Quote: |
You could use a Schottky.
I don't know enough about a PIC to be able to say for sure but on many
ADCs a high frequency high impedance input is a very bad idea.
|
Not at all- the PIC SA A/D allows the user to program the charge up time
of the S/H capacitor. So unless he there is a minimum sample rate issue,
10K is not a problem, and then only within 100mV or so of 5V out.
| Quote: |
It may be simpler to get a better op-amp but an RC between the op-amp and
the ADC may be needed.
|
Better to put the anti-alias RC before the OA IN(+)- no RC needed at A/D
input-there is no output impedance issue. |
|
| Back to top |
|
|
Ken Smith
Guest
|
| Posted:
Sun Feb 13, 2005 12:39 am Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
In article <MPG.1c785accf987f8a498968e@news.bright.net>,
Ol' Duffer <DontSend@MeSpam.net> wrote:
| Quote: | In article <culche$6jk$5@blue.rahul.net>, kensmith@green.rahul.net
says...
I don't know enough about a PIC to be able to say for sure but on many
ADCs a high frequency high impedance input is a very bad idea.
I didn't see anything about high frequency anywhere in the
thread. What are you talking about?
|
The ADC has hardware inside it that runs from the PIC's clock. The PIC
has a clock at many MHz.
[...]
| Quote: | And there is another potential problem with Bloggs' circuit.
With signals near +5V, the diode will be conducting very
little and the OA approaches open-loop conditions. You
are likely to see a lot of "fuzz" on the OA output, which
will be rectified by the diode and drive the PIC input
negative. You may need to add a few hundred pF capacitor
from OA output to inverting input to settle the thing down.
|
That capacitor will be across the diode.
I doubt that the LT081 op-amp will create enough fuzz to keep the ADC
input below the rail. As soon as it did, it would swing its output up to
some more positive voltage that would back bias the diode.
--
--
kensmith@rahul.net forging knowledge |
|
| Back to top |
|
|
Luhan Monat
Guest
|
| Posted:
Sun Feb 13, 2005 3:45 am Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
tpm@againstallspam.com wrote:
| Quote: | 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
|
LM662C - rail to rail inputs and outputs. Run it from +5 and Gnd.
--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm |
|
| Back to top |
|
|
Ol' Duffer
Guest
|
| Posted:
Sun Feb 13, 2005 5:27 am Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
In article <culche$6jk$5@blue.rahul.net>, kensmith@green.rahul.net
says...
| Quote: | I don't know enough about a PIC to be able to say for sure but on many
ADCs a high frequency high impedance input is a very bad idea.
|
I didn't see anything about high frequency anywhere in the
thread. What are you talking about?
Bloggs doesn't mention that the reason for the diode and
pullup is that you can't drive signals higher than the
PIC rail into the input switcher, or it will leak current
into the other analog inputs and you won't be able to read
them accurately.
However, I think the pullup should be 2.5K or lower, else it
will not be able to pull within half a bit of +5V PIC rail.
PIC ADC's are rather leaky and need to be driven from low
impedance source. And lower pullup resistance will make it
harder for the TL081 to pull down to 2 Volts. If it won't
make it, you could boost current with a cheap PNP transistor.
And there is another potential problem with Bloggs' circuit.
With signals near +5V, the diode will be conducting very
little and the OA approaches open-loop conditions. You
are likely to see a lot of "fuzz" on the OA output, which
will be rectified by the diode and drive the PIC input
negative. You may need to add a few hundred pF capacitor
from OA output to inverting input to settle the thing down. |
|
| Back to top |
|
|
Fred Bloggs
Guest
|
| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
Ol' Duffer wrote:
| Quote: |
And there is another potential problem with Bloggs' circuit.
With signals near +5V, the diode will be conducting very
little and the OA approaches open-loop conditions. You
are likely to see a lot of "fuzz" on the OA output, which
will be rectified by the diode and drive the PIC input
negative. You may need to add a few hundred pF capacitor
from OA output to inverting input to settle the thing down.
|
Right- if Vin is exactly 5V then any noise will cause the amplifier to
break into open loop- although the feedthrough to the output is greatly
attenuated, there is no point in having it there. A small 10uA injected
into the output node will take care of the PIC leakage with no penalty
on pull-down:
View in a fixed-width font such as Courier.
TLO8x +9V +5V
| |
| 10K
| |
+---390K---+
| |
(+) | |
Vin-------------|\ 1n4148|
| >--+--|<|--+---> TO PIC A/D
2V<Vin<5V +---|/ | |
to gnd | (-) | === |
| | 10n |
| | | |
'-----|--+--10K--'
|
|
|
------------------+--------------
common gnd |
|
| Back to top |
|
|
Fred Bartoli
Guest
|
| Posted:
Sun Feb 13, 2005 4:28 pm Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
"Fred Bloggs" <nospam@nospam.com> a écrit dans le message de
news:420EA527.9050500@nospam.com...
| Quote: |
Right- if Vin is exactly 5V then any noise will cause the amplifier to
break into open loop- although the feedthrough to the output is greatly
attenuated, there is no point in having it there. A small 10uA injected
into the output node will take care of the PIC leakage with no penalty
on pull-down:
View in a fixed-width font such as Courier.
TLO8x +9V +5V
| |
| 10K
| |
+---390K---+
| |
(+) | |
Vin-------------|\ 1n4148|
| >--+--|<|--+---> TO PIC A/D
2V<Vin<5V +---|/ | |
to gnd | (-) | === |
| | 10n |
| | | |
'-----|--+--10K--'
|
|
|
------------------+--------------
common gnd
|
This circuit will keep the TL81 output happily in its allowed range while
allowing perfect buffering from 0 to 5V.
|\ 2K2
IN >--------|+\ ___
| >+-|___|-.
.-|-/ | |
| |/ --- |
| --- 10n |
| | |
+-----' |
| ___ |
TO ADC <------+--|___|-- -'
| \ v
| R1 ---
| |
Schottky +------>|--+---- +5V
|
.-.
10K | |
| |
'-'
|
===
GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
To avoid current injection into the ADC input when the circuit clamps the
input, you either use the schottky diode (and R1=0) or set R1 to 1K or 1.2K
(and use no diode).
--
Thanks,
Fred. |
|
| Back to top |
|
|
Ken Smith
Guest
|
| Posted:
Sun Feb 13, 2005 5:45 pm Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
In article <420f2bcb$0$23451$626a14ce@news.free.fr>,
Fred Bartoli <fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
[...]
| Quote: | |\ 2K2
IN >--------|+\ ___
| >+-|___|-.
.-|-/ | |
| |/ --- |
| --- 10n |
| | |
+-----' |
| ___ |
TO ADC <------+--|___|-- -'
| \ v
| R1 ---
| |
Schottky +------>|--+---- +5V
|
.-.
10K | |
| |
'-'
|
===
GND
|
I suggest:
GND
! TL08X
GND ---!+\
! >---
GND ---!-/
!
GND
+5
! LT1498
Vin ---!+\
! >-+-- To ADC
--!-/ !
! ! !
! GND !
! !
-------
In this version the TL08X's common and output ranges have been completely
compensated for. The risk of oscillation has been reduced by the clever
use of grounding the Vcc. It takes fewer parts.
No lemmings were killed in the creation of this circuit.
--
--
kensmith@rahul.net forging knowledge |
|
| Back to top |
|
|
Fred Bloggs
Guest
|
| Posted:
Sun Feb 13, 2005 9:44 pm Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
Fred Bartoli wrote:
| Quote: | "Fred Bloggs" <nospam@nospam.com> a écrit dans le message de
news:420EA527.9050500@nospam.com...
Right- if Vin is exactly 5V then any noise will cause the amplifier to
break into open loop- although the feedthrough to the output is greatly
attenuated, there is no point in having it there. A small 10uA injected
into the output node will take care of the PIC leakage with no penalty
on pull-down:
View in a fixed-width font such as Courier.
TLO8x +9V +5V
| |
| 10K
| |
+---390K---+
| |
(+) | |
Vin-------------|\ 1n4148|
| >--+--|<|--+---> TO PIC A/D
2V<Vin<5V +---|/ | |
to gnd | (-) | === |
| | 10n |
| | | |
'-----|--+--10K--'
|
|
|
------------------+--------------
common gnd
This circuit will keep the TL81 output happily in its allowed range while
allowing perfect buffering from 0 to 5V.
|\ 2K2
IN >--------|+\ ___
| >+-|___|-.
.-|-/ | |
| |/ --- |
| --- 10n |
| | |
+-----' |
| ___ |
TO ADC <------+--|___|-- -'
| \ v
| R1 ---
| |
Schottky +------>|--+---- +5V
|
.-.
10K | |
| |
'-'
|
===
GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
To avoid current injection into the ADC input when the circuit clamps the
input, you either use the schottky diode (and R1=0) or set R1 to 1K or 1.2K
(and use no diode).
|
Very nice. No point in wasting all that A/D range- now that your buffer
can span the whole 0-5V, might as well do this:
View in a fixed-width font such as Courier.
.
. TL081
. 560K |\
. IN >--|___|-|+\ 2K2
. | >----+-|___|-.
. .-|-/ | |
. | |/ --- |
. | --- 10n |
. | | | +5 V
. | 1M5 | | |
. +--|___|----------|---+
. | | | |
. .-. | e |
. 1M0| | | \| |
. | | | 2n3906 |-'
. '-' | /| |
. | | | 5-+ - - - -------
. +---------' | | /
. | ___ | ADC | / |
. ,-----+------|___|------' IN | /
. | | 1K | / |
. | .-. | /
. | 10K| | | / |
. | | | | /
. | '-' 0-+----+--------+-----
. | | 2 5
. | ===
. | V
. '------------------------> TO ADC IN
. |
|
| Back to top |
|
|
Fred Bloggs
Guest
|
| Posted:
Sun Feb 13, 2005 10:02 pm Post subject:
Re: TL081 on Gnd/+9V, PIC on Gnd/+5V ? |
|
|
I suppose this will give a true buffer over the full 0 to 5V range on
both input and output:
View in a fixed-width font such as Courier.
.
.
.
. +9V +5 V
. | |
. | 1M5 |
. ,----------|-------|___|-----+
. | | |
. 1M0 | |\| |
. IN >--|___|---+--------|+\ 2K2 |
. | >----+-|___|-. |
. .-|-/ | | |
. TL081 | |/ --- | |
. | --- 10n | |
. | | | |
. | 1M5 | | |
. +--|___|----------|---+
. | | | |
. .-. | e |
. 1M0| | | \| |
. | | | 2n3906 |-'
. '-' | /|
. | | |
. +---------' |
. | ___ |
. ,-----+------|___|------'
. | | 1K
. | .-.
. | 10K| |
. | | |
. | '-'
. | |
. | ===
. |
. '------------------------> TO ADC
.
.
.
.
. |
. 5-+- - -------
. | /
. ADC | / |
. IN | /
. | / |
. | /
. | / |
. |/
. 0-+--------+-----
. 0 5
.
. V
. IN
.
. |
|
| Back to top |
|
|
|
|
|
|
FAQ
Memberlist
Register
Profile
Log in to check your private messages
Log in