I'm using ADM3485 and found one problem, could you please help to look into it?
The waveforms from the oscilloscope are shown as below
ADM3485E and ADM3485 Waveform , their transmitting voltage level is different.
The image you attached is not easy to see clearly, could you please update a new one? Did you test the output waveform with the same setup except the transceiver?
I have following questions that need your feedback.
What's the load conditions during the test? Were these waveform captured based on the same setup? Did the differential voltage level of ADM3485 cause communication problem? If the load resistor is 100 Ohm, the Vod could meet the spec( Table 2) in the datasheet.
Below is the customer response message
1. Experiment A, why is the voltage of A-GND and B-GND both 0.6V? When the A-B difference is 0V, the RO output is 0?
2. Experiment A is compared with D. Why do different chips have different RO, GND to A, GND to B voltage values?
3. Experiment C, we changed the KR21 and KR50 from 47K to 1K, and the RO output is 1 Can you please recommend the resistance values of KR21 and KR50?
4. At present, this problem is urgently eager for our customers. The customer has an SW9 impedance matching circuit in the RS-485 circuit. When the Receiver input of ADM3485 is open and there is no signal input, RO should output 1 (3V). But when we turn SW9 on, RO outputs 0V.
Thanks for the detailed schematic and waveform. Your query is related to the fail-safe feature, and that is actually not a problem.
I've attached the application note AN-960 here, for ADM3485/ADM3485E, both of them have open fail-safe feature which guarantees RO output high when inputs are unconnected. Based on your schematic, we see there are bias resistors network for input bus pins, and at this circumstance, the RO output should be determined by input differential voltage( VIA-VIB), to be more precisely, the triggering threshold should stay somewhere between -200mV and 200mV( Figure 10 of AN-960). Let's calculate the VIA-VIB under different bias resistors(KR21 and KR50):(Page 6 of AN960)When KR21=KR50=47K, VIA-VIB = 3.3V/(47K*2+0.12K)*0.12K = 4mVWhen KR21=KR50=1K, VIA-VIB = 3.3V/(1K*2+0.12K)*0.12K = 187mV The triggering threshold of ADM3485 should higher than 4mV and lower than 150mV( considering the 50mV input hysteresis). This explains why experiment A outputs low and experiment C outputs high. For experiment D, the triggering threshold of ADM3485E should be below 4mV, different parts might have different threshold.
If you want to keep RO output high when bus is idle, you need calculate the biasing resistor values to keep VIA-VIB>200mV.