This tool uses the following Analog-to-Digital converter specifications to find its noise figure:
- Signal-to-Noise Ratio (SNR)
- Full Scale Peak-to-Peak Voltage (VFS)
- Bandwidth (B)
The formula has been derived by Analog Devices.
NF = PFS + 174 – SNR – 10*Log10(B)
- PFS is the full scale computed from the peak-to-peak voltage
The default value for input impedance is 50 ohm. It can be changed to any value.
The input to the ADC is band-limited to half the sampling rate (fs) with a filter having a noise bandwidth equal to fs/2.
Input impedance to the ADC is equal to the source resistance. ADCs can have a high input impedance, so a termination resistance can be added in parallel external to the ADC to create the same value.
In the following example, 52.3 Ω is added in parallel to the 1 kΩ ADC impedance to create 50 Ω. (Related calculator)
Using the following parameters: peak-to-peak voltage 3.2 Volt, Sampling rate 40 MHz and Signal-to-Noise ratio 82 dB results in NF = 30 dB.
How to reduce the Noise Figure of an ADC
An RF transformer can be used to improve the Noise Figure. As Analog Devices explains in their application note, the addition of a 1:2 and 1:4 transformer improves the noise figure by 6 dB and 12 dB, respectively.
This is shown in the picture below.
Use the calculator with impedance values of 200 ohm and 800 ohm.
Note however than increasing the turns ratio reduces the full scale value and therefore the ADC range is reduced. For 800 ohm for instance PFS is reduced by 12 dB.