Crossover/Distortion on MUX

[Smillsey]

Hi all!

I have a deisgn in which i have multiple signals entering a DG1208 mux.

https://www.analog.com/media/en/technical-documentation/data-sheets/DG1208-DG1209.pdf

I am using ada4807 op amps both before (to provide gain) and after (as a buffer) the DG1208.

At the output of the DG1208 pin 8 (COM) is a non-inverting buffer as follows;

The signal going into all of the DG1208 pins isnt distorted and looks good at all frequencies (3MHz max is needed), but the signal on pin 8 above has some crossover distortion at all frequencies, and when you increase to frequencies above around 200kHz the distortion "moves" up the sinusoid .... see below

img_2956 : singals in (yellow) and out (red) of dg1208

img_2955 : closer look at the crossover distortion

img_2957 : another trace of showing the distortion

img_2958 : another look

img_2959 : Example of when frequency is significantly increased, the distortion moves along the wave, even past 90deg.

I am pondering what this is coming from, this is the first version of this board and imporvements will be made - but this distortion is baffling me a little....

The output load is minimal as the DG1208 is dirving a non-inverting buffer.... could this be the issue?

I really don't want to have to switch to using signal relays as i would need 6 and they are costly and large.

Any ideas on solutions to this would be most welcome!

 

[betwixt]

Is there any DC offset on the input voltages?
Try adding a load resistor from COM to ground (maybe a few K Ohms) to give the switch something to 'push' against. Just a guess...

Brian.

 

[Smillsey]

hey brian thanks for the suggestion... i am not super knopwledgable on these swithces.

there is no DC offset... although the next stage in the circuit is actually a DC offset stage preparing the signal for the ADC driver...

so if i cant eliminate it with a load resistor i could put this switch after the DC offset stage...

 

[KlausST]

Hi,

please show a useful schematic. .. and complete in the meaning of signal flow and parts involved.

The one in post#1 tells nothing about power supply, where the signal comes from, where it goes to, resistors, capacitors...
Also we don´t see the scope connection.


Without useful information we can´t give useful answers.

Klaus