Hi Wes, The ground plane layout will result in high speed digital currents flowing through the reference circuit. This will have some impact on the overall noise in your system. Since you added an RC on the SCLK and CONVST pins to get the communications working, this should also help with noise coupling into the reference. I suggest working your way back through your signal chain to track down where the noise is getting in (it could be in multiple places). Start by connecting VN2/VP2 in your schematic to ground to isolate the THS4521 + ADS8885 section of the circuit. If you have significant noise at this point, take a look at your power supply connections. (Make sure you are using a quiet lab supply feeding the input of the uA78M05.) You can also try reducing the resistor values by a factor of 2x (R2, R7, R9, R14). This will help reduce the effects of stray capacitance at the inputs of the THS4521 to the ground/power planes. Also, since the SN6501 is running from the same supply, I suggest disabling this section as well, just to eliminate it as a possible noise source. If this makes a significant difference, you may need to add additional capacitance on the +5VA supply, or use a different regulator. Regards, Keith
↧
