I'm driving a TXB0104 translator from 3.3V logic on the B side, VccB is 3.3V, VccA is 2.5V, OE is on VccA. Both supplies are solid. It's on a breadboard. (https://www.sparkfun.com/products/11771). The A-side gave really bad waveform at 1.5MHz while there is no load on it. All 4 channels behave the same. Is this translator broken or just not cut out for 1.5MHz?

enter image description here

  • \$\begingroup\$ The datahseet says you should expect to achieve data rates in the 20-100Mbps range. What do you have connected to the device - lots of capacitance somewhere? \$\endgroup\$ – brhans Dec 23 '18 at 3:55
  • \$\begingroup\$ Standard oscilloscope probe, 20pF. Driver side looks perfect. \$\endgroup\$ – barbazoo Dec 23 '18 at 5:18
  • 1
    \$\begingroup\$ Zout=4k after 1shot , T(decay)=800 ns, it would appear C(load)= 800n/4k = 200 pF Is probe on x1 scale?. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 23 '18 at 7:54

To allow automatic bidirectional translation, the TXB has deliberately weak output drivers so that other devices can override the voltage level.

High frequencies require high drive strength, so it has edge accelerators that increase the drive strength during transitions. To allow 100 Mbps, the accelerators are active for only about 10 ns. This means that the capacitance of your circuit must be small enough so that it can be (dis)charged in this time (see section 8.3.3 of the datasheet).

Your waveforms show that the accelerators have timed out before the voltage has reached the rails, so your breadboard, cables, and oscilloscope probes apparently add too much parasitic capacitance.

If possible, use shorter cables, and avoid adjacent breadboard contacts. If that does not help, use a different translator; if your signals are unidirectional, you can use some 74LVCxTxxx chip where you do not have drive strength compromises.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.