OPA692: Multiple output: Should no used channels need to add dummy load?

Hi DeWei,

Does any voltage noise introduced by these "antennas" indeed show up at the amplifier's low-impedance output? It seems to me that the amplifier output should provide a strong enough signal to ignore any received antenna noise from these disconnected loads. I see how adding terminations will ground any antenna formed by the loads, but it might burn additional power unnecessarily. These SMA loads are usually just used to simulate a transmission line load.

Best regards,

Sean

Hi Dewei,

DeWei Lan said:

In the some situations, I only turn CH1, CH2~4 is not used.

What do you mean by this? That you have all the shown components installed but make a cut in the copper tracks at the right crosses? And what about the cables? Have you connected a cable only to CH1 or also to CH2-4?

DeWei Lan said:

In this time, CH2~4 likes antenna.

What do you mean by this? That you fear that something is working like an antenna? Or that you noticed EMI, as if something is working like an antenna? And if you noticed EMI, where exactly did you noitice it? And how? By what sort of measurement? Again, are any cables connected to CH2-4 then?

Please be more specific. No details -> no help.

Kai

Thanks @kai klaas69 and @Sean Cashin's reply and I'm sorry reply too late.

The circuit here:

and here is portion of power

For the 1Vpp, 1M/10MHz test, they are no problem.

But when I reduce voltage to 0.1V, I discovered noise:

Above Picture: OPA3692@1MHz/1Vpp

Following Picture: OPA3692@10MHz/1Vpp

Yellow: CH2 input of Function generator (Waveform is the same to CH1 input of function generator)

Red: Output CH1 / 50 ohm load

Green: Output CH8 / 1M ohm load

I guess noise come from grounding or EMI, because when I shielding(covered) with Aluminum chassis, result is better.

So I think maybe I added SMA dummy may:

* Shielding unused output improving SNR

If I expose unused sma connector to air:

* Unused channels may be antenna if I don't connect anything.(?) EMI noise may through back with unused channels?

I know dummy load may cause additional power consumption,

but this loading is slight for my case, our power supply can provide enough current(1.5A for +5/-5V),

so that's no problem.

Hi DeWei,

This looks to me a little like amplifier instability. Adding those extra loads, should help. However, hopefully you do not require too >2.5V amplitude; the max linear output current from this amplifier is about 190mA, and excessive loading can increase the distortion of the amplifier.

Best regards,

Sean