AD8338

Hi rookie1,

The Noise Factor will depend on the source impedance and also on the gain setting used. Here is an example calculation for AD8338 at maximum gain:

https://en.wikipedia.org/wiki/Noise_figure

F= 1 + Na / (Ni * G)

Na = Output referred noise power (amplifier related)

G = Power Gain

Ni = Input noise power

Assuming Vgain = 1.1V --> 80dB voltage gain (Node Pin High) --> output referred noise = 45,000nV/RtHz --> Na = 2.03e-9 V^2/Hz

Assuming a source resistance of 50ohm --> Ni = (0.91nV/RtHz)^2 = 8.28e-19 V^2/Hz

G = (10,000) ^2 = 1e8 V^2/V^2

F = 1 + 2.03e-9 / (8.28e-19 * 1e8) = 25.4

NF = 10*log(25.4) = 14.1dB

Assuming I've not made any mistakes along the way!

You can do your calculation based on the actual gain and input / source resistance accordingly.

Regards,

Hooman

Hi,

Your answer is very helpful. I have one more question. Could you please tell me how to connect VAGC pin without using AGC feature of AD8338?

Hi rookie1,

You're welcome! :-)

Regarding your additional question on AD8338: "Could you please tell me how to connect VAGC pin without using AGC feature of AD8338?"

Datasheet page 5 states: "If the AGC feature is not used, tie VAGC to VREF".

Hopefully this answers your question.

Regards,

Hooman

Hi,

Thank you very much for your reply. According to your answer, this is the single-ended input circuit diagram after I connected it. Is there any problem?My input signal frequency is around 1kHz to 20kHz.Looking forward to your reply.

Regards

Hi,

Thank you very much for your reply. According to your answer, this is the single-ended input circuit diagram after I connected it. Is there any problem?My input signal frequency is around 1kHz to 20kHz.Looking forward to your reply.

Regards

Hi Rookie1,

Using 100uF polarized capacitors works well in simulations, but are not very practical or necessary for a 1kHz AC coupling.  Take a look at the eval board user guide for some practical values:

https://www.analog.com/media/en/technical-documentation/user-guides/UG-559.pdf

Otherwise, I think you're connecting the part up as recomended:

PB

Hi prb,

OK, I see. What capacitance should I change to?I compared them. Is there anything wrong with the circuit diagram?

Regards,

rookie