Hello, my name is Ken of Agilent Technologies.
In this video I'd like to show how to use a power supply or signal source considering the output noise.
First thing you think of when selecting a power supply or a signal source is the operation voltage range, current range and power.
For example, think of evaluating a circuit.
When you're only checking the functionality or evaluating basic performances roughly,
no other check points would be necessary.
Let's pick up a simple inverting op amp circuit to tell how to care about the noise.
This is the circuit.
The gain of this circuit is - 100.
Let's estimate the output noise.
The sources of the noise, I check usually, are the thermal noise of the resistor V1 and V2
and op amp's input voltage or current noise Vn and In here.
As far as the gain of op amp is large enough, output voltage noise can be calculated by this equation.
For the thermal noise, theoretical value is used.
And for noise of op amp, we refer to the data sheet.
For the experiment, I'll use an old op amp.
It's just in my drawer, and with numbers in its data sheet I make calculation.
This graph shows the estimated output noise calculated by using the previous equation.
You can see that the output noise and the op amp's input voltage noise effect is almost overlapped.
See the dark blue and red lines.
This means that the op amp's input voltage noise is dominant.
This is the connection diagram.
I'll use a power supply for op amp which wasn't showed in the previous diagram.
The noise of the power supply would also be a noise source.
As a rough rule, keeping noise of the power supply lower than expected output noise level would be fine.
I'll use B2962A as a power supply and connect low noise filter onto the output to reject the noise in frequency higher than 10 kHz.
This E5052B signal source analyzer is used as a spectrum analyzer.
As I also measure the output noise when applying some voltage to the input, B2961A is used as DC voltage source.
The signal and the noise of the signal source would be multiplied by 100 and will come out of the output.
This means that the noise of the signal source must be 100 times lower than the output noise.
To meet this requirement I'll use B2961A with ULNF which greatly reduces the signal source noise.
This is the inverting amplifier to be tested.
Here is the actual set-up.
You can see connections.
The input is from this B2961A
and this wires come from the B2962A as a power source.
The output is connected to the spectrum analyzer.
So, let's set + and - 15 V for power supply and start measurement.
You can check consumption current on the display of the power source.
Graph display of spectrum analyzer seems very close to the estimation.
Reading out measured data and here is the comparison graph of estimated and measured noise.
The measured value is very well matched to the estimation in the range of DC to 10 kHz.
Output noise is not affected by the power supplies and the signal source.
A little difference higher than 10 kHz occurs
because I neglected the output impedance of the op-amp and input impedance of the spectrum analyzer.
These two filters are designed and optimized to be used for B2961A and B2962A.
I recommend you to think how you need to lower the noise and choose the appropriate filter.
If you like to have more information, you can visit our website currently showing below.
Thank you.