L
Lostgallifreyan
Guest
http://preview.tinyurl.com/36vncy
Please let me know if this schematic has any glaring errors or problems.
It's a DC gain stage, with adjustable offset and scale. Precise component
values might change, but the basic form will not, unless it is bad in some
way I can't see.
The first half of the op-amp makes a new ground, so that there is enough
negative voltage to allow through-zero offset tweak to set to zero. The
supply will be a small 1 watt 5V to 24V DC-DC power converter, so that the
op-amp can have >20V supply allowing a wide range output while the supply
is only +5V. The power converter will also isolate the supply input so that
the new ground can be connected to the signal input and output ground which
will be commoned to the chassis and earth. The op-amp and the 2K2/Zener
network are both fed directly from this isolated 24VDC supply.
The second half of the op-amp amplifies a DC signal on the non-inverting
input. It's set up so the input impedance is as high as possible, but a
resistor might be placed across the input in any finished circuit.
When I built this, it worked accurately and quietly on a pinboard, and on a
small PCB I made for it, when reading the output on a Fluke voltmeter, but
when wired to an LED panel meter in a small metal box the reading was
erratic. I traced this to the meter changing its current draw when
different segments lit, and the ground circuit current seemed to be
affecting the gain stage output, causing a feedback effect that I couldn't
eradicate, no matter how I arranged the ground wiring.
Is there something about this circuit that is inherently unstable or
inaccurate? If so, what should best be changed?
Please let me know if this schematic has any glaring errors or problems.
It's a DC gain stage, with adjustable offset and scale. Precise component
values might change, but the basic form will not, unless it is bad in some
way I can't see.
The first half of the op-amp makes a new ground, so that there is enough
negative voltage to allow through-zero offset tweak to set to zero. The
supply will be a small 1 watt 5V to 24V DC-DC power converter, so that the
op-amp can have >20V supply allowing a wide range output while the supply
is only +5V. The power converter will also isolate the supply input so that
the new ground can be connected to the signal input and output ground which
will be commoned to the chassis and earth. The op-amp and the 2K2/Zener
network are both fed directly from this isolated 24VDC supply.
The second half of the op-amp amplifies a DC signal on the non-inverting
input. It's set up so the input impedance is as high as possible, but a
resistor might be placed across the input in any finished circuit.
When I built this, it worked accurately and quietly on a pinboard, and on a
small PCB I made for it, when reading the output on a Fluke voltmeter, but
when wired to an LED panel meter in a small metal box the reading was
erratic. I traced this to the meter changing its current draw when
different segments lit, and the ground circuit current seemed to be
affecting the gain stage output, causing a feedback effect that I couldn't
eradicate, no matter how I arranged the ground wiring.
Is there something about this circuit that is inherently unstable or
inaccurate? If so, what should best be changed?