If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

# AFE5816: the noise requirement of the CW summing amplifier

Part Number: AFE5816
Other Parts Discussed in Thread: THS4130

Dear team,

In the datasheet, you have mentioned that "The amplifier noise and matching have a direct impact on the I/Q channel performance and therefore must be selected cautiously. Amplifiers with input-referred voltage noise lower than 2 nV/√Hz can be selected.".I guess that you came out this number from the Vn,ircw.  But I still don't know how you came out the number of 2nV/sqrt(hz).

Could you please simpliy demonstrate the calculation about how you get the number of "2nV"?

Best Regards

Zhihong Huang

• Hi Zhihong,

Thanks for reaching out to us.

Let me explain the reasoning behind selecting the noise specification of the external current amplifi-er.

As shown in the figure below, the CW outputs are current output and the corresponding signal chain looks like below which is comprising of 16 LNAs.

Calculating the Noise at the output of the device.

Say, LNA gain is 18dB.

The input referred Noise of at the input is 1.55nV/rtHz.

So the noise contribution due to LNA will be: 1.55nV/rtHz x 7.93 x sqrt(16) /16 =  3.07 nV/rtHz --- (1)

Noise contribution due to the each 500 ohm resistor will be 2 nV/rtHz

So the noise contribution due to resistor will be: 4 nV/rtHz x sqrt (16)/16 = 1 nV/rtHz ----  (2)

Total Noise (1) + (2) : 3.3 nV/rtHz --- (3)

Now in order to match the swing, Rext should be 500/16, for 2Vpp swing:

Hence Noise at the output will be : 3.3 (1+1) = 6.6 nV/rtHz

From equation 3.

Hence, in order for signal noise to dominate, the input referred noise of the amplifier should be kept at least 3 dB lower and hence the number roughly comes out to be close to 2 nV/rtHz.

I hope the above explanation should help you understand the specification placed on the external amplifier.

Thanks & regards,

Abhishek

• Dear team,

Thanks for your reply. It is quite specific but I still have one question:

" Now in order to match the swing, Rext should be 500/16, for 2Vpp swing: Hence Noise at the output will be : 3.3 (1+1) = 6.6 nV/rtHz ,3.3 (1+1) = 6.6 nV/rtHz"

Why should the Rext be considered in the noise caculation? I think from your calculation the noise requirement of the amplifier should be directly equal to :3.3/1.414 = 2.33. Am I rignt?

Best Regards

Zhihong Huang

• Hi Wesley ,

You are right.

For external amp noise specification. Rext doesn't matter.

I just calculated the total noise at the final output.

Regards,

Abhishek

• Dear team,

If I use the 8 AFE for the 128 channel application as shown below, I think the noise requirement for FDA should be the same results as you posted. Am I right?

Best Regards

Zhihong Huang

• Hi Zhihong,

Yes, i think it should be similar. But just in case, I will check with my team and get back to you early next week.

Just ensure the current handling capability of the THS4130 is within the requirements.

Thanks & regards,

Abhishek