J
James A
- Jan 1, 1970
- 0
I'm using a DC-DC converter to step down (4-4.8V) downto 3.5V to drive
a number of Lumileds. The voltage source is a battery. (4.8V -> 4V)
On the output I want to sense the voltage to be able to control the
duty cycle. I'm using a PIC to monitor it as I also want to record
some of the voltages.
To sense the voltage I decided to use an Op-amp. I hooked it up in a
Voltage follower configuration. I figured the low output impedance
would result in a shorter aquisition time for the a/d converter.
I thought that the gain would be slightly less than one, boy was I
wrong.
I tried a number of different voltages and received the following
results:
Input | Output
4v - 2.91
5V - 3.83
10V - 8.93
20V - 18.88
30V - 28.8
As the supply voltage increased the gain got closer to unity. This is
of no use to me given that i will be operating at sub 5V levels.
Also, why is the difference always about ~1.15
The Op-amp used: LM358
Reasons:
-Cost
- No need for dual power supply
- Drain suitable for battery powered operation
I also then went on and tried the LM324 and received almost identical
results.
I'm certain someone can suggest a cheap method of monitoring the
voltage, is the op-amp the way to go?
If so, what specs should i be looking for?
Cheers,
James A
a number of Lumileds. The voltage source is a battery. (4.8V -> 4V)
On the output I want to sense the voltage to be able to control the
duty cycle. I'm using a PIC to monitor it as I also want to record
some of the voltages.
To sense the voltage I decided to use an Op-amp. I hooked it up in a
Voltage follower configuration. I figured the low output impedance
would result in a shorter aquisition time for the a/d converter.
I thought that the gain would be slightly less than one, boy was I
wrong.
I tried a number of different voltages and received the following
results:
Input | Output
4v - 2.91
5V - 3.83
10V - 8.93
20V - 18.88
30V - 28.8
As the supply voltage increased the gain got closer to unity. This is
of no use to me given that i will be operating at sub 5V levels.
Also, why is the difference always about ~1.15
The Op-amp used: LM358
Reasons:
-Cost
- No need for dual power supply
- Drain suitable for battery powered operation
I also then went on and tried the LM324 and received almost identical
results.
I'm certain someone can suggest a cheap method of monitoring the
voltage, is the op-amp the way to go?
If so, what specs should i be looking for?
Cheers,
James A