Hi,

I am using AD8338 VGA in a non-AGC application (DETO pin connected to COMM, AGC connected to VREF) with my input voltage source connected at INPD and INMD via 1.2Kohm resistors (INPR connected to INMR).

I wish to calculate the input referred noise of my circuit. Do I read the input referred noise of AD8338 directly from the input referred noise graph in the data sheet, or do I have to work backwards from the data sheet output referred noise using the gain of my particular setup?

Also, does enabling the OFSN pin affect noise levels?

Thanks,

joman

Hi joman,

Apologies for late response.

When using AD8338 configured above, you will expect the following:

At a maximum gain of 80 dB, AD8338 has an Input-referred noise of

4.5 nV/√Hz. The two 1.2 kΩ will contribute a noise of approximately6.3 nV/√Hz. Taking the RSS if the two noise, the Total Input-Referred Noise that you would expect in the input is7.75nV/√Hz.There will be a trade off in the range of AD8338 but the noise will reject it. AD8338 follows this gain equation:

Gain(dB) = 80*Vgain + 20log(9500/Rin) - 34.

For the 500 ohms input resistors, the gain is 80 dB. For 1.2 kΩ resistors it would be ~72 dB. The 72 dB gain only applies to the signal but the noise still sees a 80 dB gain. This means that you would still expect 7.75 nV/√Hz * 10,000 = 77.5 uV/√Hz noise in the output. Assuming, we have a 1kHz bandwidth filter, the rms noise would be 2.5mVrms. With a maximum linear output voltage of 1Vrms, the SNR is 52 dB. Given all these, the largest input signal would then be 1Vrms/3970 V/V = 252 uVrms.

I hope this helps.

Regards,

Jino