ALM2402F-Q1: Output in HiZ connected to another output

Hi Anders,

Q: - Will this work only by setting ALM2402 in SHDN mode, - Do I need to turn off the VS_O supply

OTF/SH_DN bit information is shown below. You can control the state of ALM2402 with the single bit. I don't think that you need to power cycle the VS_O pins. 

If I remember correctly, RS485 is a serial communication protocol, and it sends information in bytes, words etc.. How are you going to control the bit state directly? Unless you have a transceiver to convert a command to single bit. 

Best,

Raymond

Hi Raymond and thanks for the reply.

Yes, I have a transceiver on the board already. The thing is that we have a board designed for an encoder. The encoder has both quadrature signals and an RS485 communication. The board connector do not have any spare pins so I need to use the same pins for resolver excitation as the RS485 interface. The transceiver tolerates the high voltage created by the resolver excitation so that is no problem. My concern is if there will be problems with the HiZ outputs of ALM2402 when an encoder is connected talking RS485.

/Anders

Hi Anders

Q: My concern is if there will be problems with the HiZ outputs of ALM2402 when an encoder is connected talking RS485.

I do not know what is the output stage looks like at the transceiver. If you are concerned about digital signal interference, maybe you can put an opto or something similar (filter) to isolate the possible digital switching interference. You mentioned that RS485 is running at 9600 bps. 

Below is the summary of the OTF/SH_DN bit datasheet. 5Vdc was used to pull up the pin with 2.5kOhm pul-up resistors during the lab characterization, so that is approx. 5V/2.5kohm or 2mA. 

If you have other questions, please let us know. 

Best,

Raymond

Anders,

The entire idea behind SHDN mode of ALM2402F-Q1 is to allow exactly what you are trying to do - a shutdown pin pulled low makes the output high impedance. And as long at the transceiver’s signal does NOT drive the output of ALM2402F beyond its rails (ALM2402F will be 9.5VDC supplied and the transceiver signals will be 0-5 V, 9600 baud) the output may see at most a feedback resistor as its load and thus should be able to be driven by transceiver. 

However, there is one exception: because there are back-to-back input protection diodes between the input terminals, in case of a buffer configuration you may have to add 500ohm feedback resistor to limit the current to 10mA or less - see below.