We designed a PCB with an ADM2582E onboard as the RS485 transceiver for our customer, but got feedback w.r.t EMI issues of our design. As we don't have a lot of experiences on this matter, we read the application notes AN-0972 and AN-1349, which helped quite a lot. We also bought an ADM2582E EMI evaluation board EVAL-ADM2582EEBZ as the reference for our design revision and tests.
However, when we were doing EMI tests with the EVAL-ADM2582EEBZ board in the last few days, we couldn’t reproduce similar results shown in the EVAL-ADM2582EEBZ user guide (UG-044). The settings we used for the board are as follow:
- 9V battery for the onboard regulator ADP667, and 3.3V VDD for ADM2582E
- ADM2582E’s driver and receiver are both enabled
- 8MHz TxD signal from onboard oscillator LTC6900 (to do this, we connected the Pin4 ‘DIV’ of LTC6900 to ground and replaced the resistor between Pin1 ‘V+’ and Pin3 ‘SET’ with a 25kΩ one to get 8MHz signal out of LTC6900)
- Y, Z are connected to A,B respectively (Jumpers LK5, LK7 are shorted)
- Two 120Ω termination resistors are fitted at RT and RT1 footprints
EMI tests were carried out in a 3-meter semi anechoic chamber, and the results we got is shown below, where we can see the emission levels near 180MHz are still quite high, and lots of small peaks across the whole 30MHz to 1GHz frequency range which we suspected come from the 8MHz oscillator.
The result shown in the UG-044 Figure 6 is much much better, especially at near 180MHz range where there is almost no very obvious high emissions. (Though it seems that there is a problem with Figure 6. As Table 6 says it's 37dBuV/m at around 358MHz, in the Figure 6 the peak point at this frequency is lower than 30dBuV/m.)
We’re tring to figure out why we cannot get similar results before we go on to revise our own PCB. Is there something we missed or something we’re not doing right? Could you help? Thank you!