Hi,
Because I have some time left today, I want to show you another project I designed and built to measure the noise / PARD level of power supplies and also voltage references.
But not intended for measuring the best voltage references, this amplifier is not good enough for that.
First take a look at what the amplifier looks like after it's finished.
Functional schematic
And this is the schematic
Architect specificationsPower: 2x9V battery
Input and Output protection +-60V
Compact
RI > 20K
Rout 50 Ohm, unterminated 1M cable
Reasonable immunity to LAB Bench EMC
Switchable High Pass input filter
Switchable 2x Low Pass filter, 10KHz and 100KHz
Perfect square wave reproduction
No special components
Low distortionAs can be seen in the diagram, the amplifier is constructed with two opamps.
After some testing I chose the LM4562 because of the good mix of properties for my application.
TI has stopped producing this IC but still has a lot of work to do here in Europe.
There is also a replacement product available with better specifications in some areas.
Both amplifier steps have an HF trim function for the perfect square wave reproduction I wanted.
I have selected the resistors that set the gain, so that the gain is within a few tenths of a dB.
Where necessary, I have built up the circuit with low capacity for the best frequency response.
The whole circuit is built on the well-known green Chinese circuitsboards and so where it maked sense,
for example at the inverting input of the opamps I have drilled out holes.
There are many opamps usable for this application, but almost all of them have the annoying feature that the pulse response depends on the signal level.
The LM4562 is one of the few exceptions and that is why it is used in this circuit.
Some pictures of building the amplifier
Drilled out holes
Also on this sensitive opamp input...
On top the capacitor bank for the input and at the bottum the capacitors for the output
Some of the first test are done here.
square wave respons of a 10KHz signal.
And now at 100KHz.
1MHz!
A nice fast symetrical puls.
The bottomview
He is finished!
Testsetup outside the LAB for noise measuring
A -40dB reference signal and the noise from the amplifier in the low frequencies.
6Vtt a upgoing signal (10KHz)
a 1.2V signal, this is wat im mean, no abberation
Old Scool frecuency respons...
400HZ Hig Pass filter.
10KHz and 100KHz filter respons.
Some distortion measurements done on a Audio Precision.
1V RMS Out 1KHz and 22KHZ bandwith.
And now at 4V RMS output level same filter conditions, lower distortion because of the bigger level noise distance.
Same 4V RMS level, but now a 10KHz signal en the filtering is at 80KHz, i do not complaine..
Question, what is the cause of the extra signal on this pulse.
And last picture, the simple low batt indicator that is used.
Shoot at it!
Kind regards,
Bram