THVD1452: Receiver Input Impedance

The load is 1/8th of an RS-485 unit load.

The behaviour is not exactly like a fixed resistor, which is why this is specified as the bus input current in section 7.7.

If you really need an input impedance value, divide V_I by I_I.

Hi Shinichi,

Clemens is correct - the load is rated as a 1/8th unit load - and for general design purposes we use 96k as the minimum impedance. In most cases for 1/8th unit load devices the impedance is different, but for most designs using 96k for 1/8th load will be fine - if you need more specific boundary points the bus input current spec divided by the input voltage will give you impedance values that can be design with. However for most applications this is unnecessary as 96k was probably a fine value design with.

As a quick tool to understand RS-485 transceiver impedance quickly is:

1 unit load ~ 12K ~ supports 32 nodes

1/2 Unit load ~ 24k ~ supports 64 nodes

1/4 unit load ~ 48k ~ supports 128 nodes

1/8 unit load ~ 96k  ~ supports 256 nodes

The 12k comes from the minimum common mode loading spec for RS-485 which is 375.  32 12K loads in parallel is 375 - the same analysis can be used for the other values as well - but just to give a bit of information where the numbers are coming from. 

Please let me know if you have any other questions and I will see what I can do. 

Best,

Parker Dodson

The RS-485 specification defines a unit load not as a fixed resistance, but with several limits on voltages and currents (see figure 4-3 of the RS-485 Basics Series whitepaper).

This can be approximated with 12 kΩ, but at other places in the graph, the computed impedance would be higher or lower.

Every RS-485 transmitter is guaranteed to be able to drive a bus with 32 unit loads (or 256 1/8th unit loads); this is why the datasheet tells you that the THVD1452 is 1/8th unit load, and why you should use unit loads to design a bus. You can also use approximate impedances (375 Ω, 12 kΩ, 96 kΩ) to design a bus; the resulting maximum number of bus nodes is the same, even if the impedance valus are not actually exact for all voltages.

Hi Shinichi,

Sorry I wasn't as clear. RS-485 as a standard is full of assumptions that can blunt the nuance of the system - Clemens explanation is also correct too - so that can also be used to add some context.

When I say "minimum" input impedance - its not really minimum - it can be referred to that because of how the transceivers are specified. The differential bus specifications will include a common mode loading - i.e. there is a resistor from A  and B to a common mode source (usually spec'd -7V to 12V) where the common mode resistor is 375 Ohms.  That is the smallest impedance that is used as a common mode load that is specified in our datasheets (and most industry datasheets). For 256 Transceivers in parallel - for them to equal 375 they would need an impedance of 96K - so for 1/8th load devices we approximate. the "minimum" input impedance to be 96K. Even though each receiver can be lower than 96K.  In reality - on a long bus with multiple devices (up to 256) each transceiver is going to have a different input impedance that is largely dependent on the difference in ground potential between nodes - so its easier to design with 1 value - and for a 1/8th unit load device that is generally pegged at ~96K. 

But 256 1/8th unit load devices can be on 1 bus - regardless if some of the nodes are below 96K during operation - as that is specified and is possible. The only caveat here is that the true common mode loading may be slightly less than 375 Ohms - but in the vast majority of applications this shouldn't be a concern. 

Also another important note - a lot of reference material and design guides assume the 12K | 24K | 48K | 96K for 1 , 1/2, 1/4, and 1/8 unit load devices.

So in conclusion:

96K isn't the true "minimum" but for design purposes it can be used that way (and a lot of reference material is also going to reference it this way as well) - the  main exception I would say is:

You have a bus approaching the 256 node limit with a large amount of ground potential differences between nodes possible  - if this is the case an understanding of the ground potential differences seen between nodes is important so that the design can better approximate the true common mode loading of the bus to see if they should expect deviation in datasheet values due to slightly lower than 375 Ohm common mode loading - once again - it probably isn't going to be a huge concern in most applications, but something that is good to be aware of.. 

Best,

Parker Dodson