0
\$\begingroup\$

I need to drive a TTL signal into a 50ohm load (connected to a 5V supply, not to the ground) from a DAQ card that can only output 5mA. I therefore looked around and found Linear Technology has LT1010, which is a buffer with 150mA current capability.

It should be very simple to use, but when trying to simulate it using LTspice, I get very weird results. I've attached a picture of my schematic (sorry it's ugly, was supposed to be a quick and dirty simulation): Schematics

When I set the input to 5V, everything is OK. But when I set it to 0V, The output is reaching 1.2V.

The bias 20ohm resistor can be changed to any value from 0 (no resistor) to 1k and it doesn't change anything, I've also tried adding a negative supply to the V-, but it doesn't change much.

According to the LT1010 spec, it should be able to drive a 150mA load, doesn't it means that the output should follow the input (up to a small offset) up to 150mA?

\$\endgroup\$
2
  • \$\begingroup\$ How fast of a signal do you need to drive? Can you get away with 'jellybean' rail-to-rail op amp? \$\endgroup\$
    – Nicholas Clark
    Commented Jul 19, 2016 at 16:00
  • \$\begingroup\$ It's pretty fast, I need at least 100MHz. I'm sorry but I'm not familiar with the terminology, what's a 'jellybean' rail-to-rail op amp? (just the jellybean part I don't understand :)) \$\endgroup\$
    – Yomach
    Commented Jul 20, 2016 at 7:20

2 Answers 2

Reset to default
4
\$\begingroup\$

Look at the negative saturation voltage curve in the datasheet. If you operate this part with a single supply, the output won't be able to swing to ground. At 25C with 150mA load you will only be able to get down to about 2.9V. This isn't really meant for digital applications like this, you should look for a high current digital buffer or maybe a discrete circuit.

enter image description here

\$\endgroup\$
1
  • \$\begingroup\$ Ahh, I see, thanks! I'll look for a digital buffer or just build my own with a fast rail-to-rail op-amp. \$\endgroup\$
    – Yomach
    Commented Jul 20, 2016 at 7:21
0
\$\begingroup\$

Welcome to EE.SE. The LT1010 is not a rail-to-rail device, so it is unable to output zero volts for zero volts input, if powered from +something and zero volts.

If you add a second voltage source, from ground to a negative voltage, say -5v, and feed this to the LT1010, it should work as expected.

\$\endgroup\$
4
  • \$\begingroup\$ Is the LT1010 obsolete? \$\endgroup\$
    – Robert Endl
    Commented Jul 19, 2016 at 15:03
  • \$\begingroup\$ There is no mention of it on their page at this time, but I do not know with certainty. \$\endgroup\$
    – rdtsc
    Commented Jul 19, 2016 at 15:12
  • \$\begingroup\$ I found it...one package went obsolete (can). Never mind. \$\endgroup\$
    – Robert Endl
    Commented Jul 19, 2016 at 19:01
  • \$\begingroup\$ Ahh, I see, thanks! It seems that even with a -5V supply it only reaches 0.4V. I might just build my own with a fast rail-to-rail op-amp. \$\endgroup\$
    – Yomach
    Commented Jul 20, 2016 at 7:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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