W
Walter Harley
- Jan 1, 1970
- 0
(...I know, you read the subject line and you think "duh, of course it is.")
I'm having an unexpectedly hard time getting a comparator circuit to work,
and would appreciate a bit of insight into what I'm doing wrong.
The context is that I'm trying to design a circuit to detect presence of a
musical instrument signal and turn on the power to a battery-powered device.
So, I want a circuit that will run on 9v, with max current draw of less than
20uA or so. It needs to turn "on" with signal levels of as low as 20mV
peak. The input impedance needs to be at least 1MEG, hopefully higher.
Frequency response can be awful: it's fine, even desirable, to only detect
signals from 50Hz to 2kHz.
So, that was the context. Here's the present problem. I've got a very
simple comparator circuit, that is the first stage of this control device.
And I can't keep it from oscillating, around 1MHz, no matter what I do.
Below is one version of the circuit. I've played with all the values, with
no luck. I've put bypass capacitors everywhere I can think of, including
the + input of the comparator (but that one was only 10nF). I've run it off
bipolar supplies, even though the TS393's common mode range includes ground.
All leads are short, but it is breadboarded on one of those spring-clip
breadboards.
I assume that if I got my resistances low enough I would swamp whatever
capacitive effects are causing this. But the whole point of this exercise
is to be low power. I assume that other people are using micropower
comparators successfully... what am I missing? Do I need to actually build
a PCB with guard traces, or air-wire the whole thing, before I know whether
it works?
Thanks!
VCC
+
|
o-----o---o--------.
| | | |
.-. | | .-.
3.3M | | | | | |
| | | | 10k| |
'-' | | '-'
| | | |
| |\| TS393 |
o---|+\ | |
| | >--|--------o
.------o----------|-/ | |
|1M | | |/| | |
.-. | | | | |
| | 47pF | | | ___ |
| | | o-----|---|-|___|--'
'-' | | 3.3k| | 3.3M
| --- .-. | |
| --- | | | ---
| | | | | ---
| | '-' | | 0.1uF
| | | | |
o------o------o-----o---'
|
===
GND
created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de
I'm having an unexpectedly hard time getting a comparator circuit to work,
and would appreciate a bit of insight into what I'm doing wrong.
The context is that I'm trying to design a circuit to detect presence of a
musical instrument signal and turn on the power to a battery-powered device.
So, I want a circuit that will run on 9v, with max current draw of less than
20uA or so. It needs to turn "on" with signal levels of as low as 20mV
peak. The input impedance needs to be at least 1MEG, hopefully higher.
Frequency response can be awful: it's fine, even desirable, to only detect
signals from 50Hz to 2kHz.
So, that was the context. Here's the present problem. I've got a very
simple comparator circuit, that is the first stage of this control device.
And I can't keep it from oscillating, around 1MHz, no matter what I do.
Below is one version of the circuit. I've played with all the values, with
no luck. I've put bypass capacitors everywhere I can think of, including
the + input of the comparator (but that one was only 10nF). I've run it off
bipolar supplies, even though the TS393's common mode range includes ground.
All leads are short, but it is breadboarded on one of those spring-clip
breadboards.
I assume that if I got my resistances low enough I would swamp whatever
capacitive effects are causing this. But the whole point of this exercise
is to be low power. I assume that other people are using micropower
comparators successfully... what am I missing? Do I need to actually build
a PCB with guard traces, or air-wire the whole thing, before I know whether
it works?
Thanks!
VCC
+
|
o-----o---o--------.
| | | |
.-. | | .-.
3.3M | | | | | |
| | | | 10k| |
'-' | | '-'
| | | |
| |\| TS393 |
o---|+\ | |
| | >--|--------o
.------o----------|-/ | |
|1M | | |/| | |
.-. | | | | |
| | 47pF | | | ___ |
| | | o-----|---|-|___|--'
'-' | | 3.3k| | 3.3M
| --- .-. | |
| --- | | | ---
| | | | | ---
| | '-' | | 0.1uF
| | | | |
o------o------o-----o---'
|
===
GND
created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de