Other Parts Discussed in Thread: SN65HVD82
I have found a lot of incosistent document in this theme. I would like to clarify them.
1. First time, I need have clarify the typos in documents. Please confirm that TI's guide has a lot of typos.
Table 1:
"–0.2 V < VAB < 0.01 V" => "–0.2 V < VAB < -0.01 V"
"–0.2 V < VAB < 0.07 V" => "–0.2 V < VAB < -0.07 V" by Vit+,max + Vhys,min at Z-gen
"which is almost three times the noise immunity of legacy devices with external biasing." => I can not see this in example of Figure 1.
What I see
X-gen: it has Vnoise,pp=200mV, at Vhys,min=50mV; Vit+=+200mV; Vab,in=+250mV biasing => (50+50)*2
Y-gen: it has Vnoise,pp=170mV at Vhys,min=35mV; Vit+=-10mV; Vab,min=+40mV biasing => (45+40)*2
Z-gen: it has Vnoise,pp=140mV at Vhys,min=50mV; Vit+=-20mV; Vab,min=N/A => (20+50)*2
Of course, I agree, everything depends on that what Vhys,min means at boundary of 200mV. In my terminology, receiver output is not changed until voltage level reaches the level of "Vit+ - Vhys"
2. Is THVD1410 included in Z-generation group?
DS says here: THVD14xx 3.3-V to 5-V RS-485 Transceivers with ±18-kV IEC ESD Protection datasheet (Rev. E)
"When the differential input signal is close to zero, it is still above the VTH+ threshold, and the receiver output will be High. Only when the differential input is more than VHYS below VTH+ will the receiver output transition to a Low state. Therefore, the noise immunity of the receiver inputs during a bus fault conditions includes the receiver hysteresis value, VHYS, as well as the value of VTH+."
3. Is SN65HVD3082 included in Y-generation group?
"When the differential input signal is close to zero, it will still be above the VIT+ threshold, and the receiver output is High. Only when the differential input is more negative than VIT– will the receiver output transition to a Low state. So, the noise immunity of the receiver inputs during a bus fault condition includes the receiver hysteresis value VHYS (the separation between VIT+ and VIT– ) as well as the value of VIT+."
Maybe this is typo: "Only when the differential input is more negative than VIT–" => Are you thinking "Only when the differential input is more negative than VIT+-VHYS (VHYS below VIT+)"?
4. SN65HVD72DR is Z-gen part (said by Ti's guide). It is correct definition and picture.
"When the differential input signal is close to zero, it is still above the maximum VIT+ threshold of –20 mV, and the receiver output will be high. Only when the differential input is more than VHYS below VIT+ will the receiver output transition to a low state. Therefore, the noise immunity of the receiver inputs during a bus fault condition includes the receiver hysteresis value, VHYS, as well as the value of VIT+."
Figure 22.
5. SN65HVD82DR is Z-gen (said by Ti's guide)
DS says: SN65HVD82 Robust RS-485 Transceiver datasheet (Rev. B) (ti.com)
"When the differential input signal is close to zero, it will still be above the VIT+ threshold, and the receiver output will be High. Only when the differential input is more negative than VIT– will the receiver output transition to a Low state. So the noise immunity of the receiver inputs during a bus fault condition includes the receiver hysteresis value VHYS (the separation between VIT+ and VIT– ) as well as the value of VIT+."
You can see, that definition the same with SN65HVD3082, but Figure 20 description is opposite with this.
Second, typical and worst case minimum values of Vhys are mixed. It is very confusing.
SN65HVD82 Robust RS-485 Transceiver datasheet (Rev. B) (ti.com)
This part has value of 40mV for Vhys,min. So this part has only noise immunity of 120mV. SN65HVD72DR is better in real life.
References:
RS-485 failsafe biasing: Old versus new transceivers (ti.com)
THVD14xx 3.3-V to 5-V RS-485 Transceivers with ±18-kV IEC ESD Protection datasheet (Rev. E)
SN65HVD7x 3.3-V Supply RS-485 With IEC ESD protection datasheet (Rev. H)
SN65HVD82 Robust RS-485 Transceiver datasheet (Rev. B) (ti.com)
I look forward to your comments.
Attila