3
\$\begingroup\$

This is my first time posting on this forum, so please bear with me. I'm trying to design a 'logic converter' to convert a signal from one logic standard to another, and vice-versa. The first logic standard is the Nuclear Instrumentation Module fast logic standard (NIM logic), whose logic voltages are 0 volts when low and -0.8 volts when high, and it remains high for only 10 nanoseconds. The other logic standard is the TTL standard, 3.3 V when high and 0 V when low. The inputs can also be seen as 50 Ohm line outputs depending on the direction. In terms of power supplies available, the module that will power this circuit has +6, 0 (GND), and -6 volts to work with.

I'm trying to start off by converting the NIM logic to TTL logic. My thought was that I can first boost the NIM logic signal to something higher, and then use that larger voltage to activate a switch to provide the 3.3 V output as needed. For example, I boost the input so it reaches 0 V and -1.8 V, which then activates a PMOS switch to provide a 3.3 V signal accordingly. The issue is that with such a small input voltage, it's not enough to activate a PMOS (specifically the ALD 1107), so I thought I had to use BJTs to get the fast switching necessary. I tried simulating it with a current switch (emitter coupled), but it doesn't seem to reach the appropriate low logic level needed.

schematic

simulate this circuit – Schematic created using CircuitLab

The circuit shown is what I have so far, and I'm simulating it through LTspice. I have very little experience designing circuits like this, and I'm not sure what else I can do to approach this problem. The time to switch is in nanoseconds, and I'm not even sure if my choice of BJTs is appropriate for the time needed. Power isn't a problem right now; I'm just trying to see what I can do to get it working. Are there any other kinds of circuits that would be helpful to look at given the goals?

\$\endgroup\$
  • \$\begingroup\$ What speeds are you trying to accomplish? \$\endgroup\$ – Scott Seidman Jan 17 '17 at 15:55
  • 1
    \$\begingroup\$ If BJT's are your preference, you might follow the topology (or even adapt) a ECL-to-TTL translator. You might find this guide useful:onsemi.com/pub_link/Collateral/AN1672-D.PDF \$\endgroup\$ – glen_geek Jan 17 '17 at 16:31
  • \$\begingroup\$ @ScottSeidman Ideally, I would like to achieve the same speeds as the input NIM signal so that we can get the TTL signal at the same time and duration as the NIM signal, though considering how this can't be ideal, some delay would have to occur as long as the TTL signal can be received at the same time or close to as the NIM signal. \$\endgroup\$ – user101402 Jan 18 '17 at 16:40
3
\$\begingroup\$

Firstly, you seem to be trying to duplicate what already exists in the right format for NIM. Converters such as this provide both NIM-->TTL and TTL-->NIM with the right cabling for most crate instruments.

If you want to do it yourself, your might consider something like this for NIM-->TTL:

Trying to convert 10 nS pulses is quite a challenge and I'd suggest you treat the interface as simply a linear analog problem. You have a 10nS pulse (with probably t(r) and t(f) times of about 2.5-3 nS) and simply need to convert this to TTL voltage levels (I'd suggest the first TTL input devices you touch be 74Sxx, although you might be able to use 74HCT).
I'm assuming your signal is -800 mV on a cable terminated (50 Ohms) at both ends.

schematic

simulate this circuit – Schematic created using CircuitLab

This has a protected front end, but to do so you need to use rather special diodes, the LL101A's have 1 nS recovery times and less than 2 pF capacitance.
The OPA355 has about 300V/uS slew rate and can work nicely over the 3.3 V to -1400 mV supply rails to avoid having to offset the inbound signal at all.
The OP355 also has sufficient drive current to drive 50 Ohm for a TTL-->NIM conversion, which you could do in a very similar fashion.

Why the special diodes To provide protection on the input you need to add series resistance. This does not impact the 50 Ohm termination, but allied with any input capacitance for the amplifier (whether an op-amp or transistor) will form a low pass filter on the input signal. The diodes chosen have about 1.5 pF at zero volts, and the amplifier has about 1.5 pF. This gives a low pass filter with a maximum risetime/falltime of < 4 nS. This could be reduced by reducing the 560 Ohm series resistor. For example lowering it to 100 Ohms provides a risetime of < 1 nS. You can calculate the risteime using an online calculator like this which shows group delay and phase too.

Why the high slew rate amplifier To translate the inbound pulse to TTL and keep somewhat the same pulse width, tr and tf is hard. For the TTL signal you need a fast risetime between V(inputLo) and V(inputHi) for whatever device you are driving. The faster the slew rate the better you can follow the input signal. If the input signal risetime is say 3 nS then to achieve the same (10%-90%) transition for a 74S04 you have to transit from 0.8 V (V(Lo)) to 2 V (V(Hi) or 1.2 V. 1.2 V in 3 nS is roughly 1200 V/ uS. This amplifier only achieves about 300 V/uS, which may be ok for NIM counting applications, but would certainly be a concern if you were looking at signal phase relationships in the 100 Mhz region.

\$\endgroup\$
  • \$\begingroup\$ Thanks for your answer, and your circuit. I know that there are modules that exist; I just thought that it would be cheaper if I tried to create a circuit on my own for this rather than purchasing a module. I thought that it could be accomplished using transistors rather than just using an op amp outright, but I'll look at your circuit. Sorry, I'm unfamiliar with the nuances of op amps. All I'm comfortable with is an ideal model, so I'll look up some details in your answer. But, why are the special diodes needed? If it's just for circuit protection, wouldn't regular Zeners work? \$\endgroup\$ – user101402 Jan 18 '17 at 16:46
  • \$\begingroup\$ The special diodes require very low capacitance. The BAT54's although called a fast recovery diode (5 nS) have 15 pF at zero volts so can't be put in the signal path. Added to the input capacitance of the OPA355 that gives you about 17 pF on the input. This is significant enough to slow risetimes and impact your pulse width. I'll add some detail to the answer. \$\endgroup\$ – Jack Creasey Jan 18 '17 at 17:25
2
\$\begingroup\$

I would suggest an LVDS receiver for this application.

Terminate and bias the input voltage so that it's always positive and 'sees' 50\$\Omega\$ (2 resistors) and bias the other input at the mid-point between on/off (2 more resistors). Add one 35 cent chip and you're done.

Common chips can handle 400Mbaud, and have a couple nanosecond propagation delay so they should be fine to detect a 10ns pulse.

\$\endgroup\$
0
\$\begingroup\$

Three designs: 1) Common_Base is topology to use. Get a 2N3904 (NPN) or something a bit faster. Ground the base. Put 50_ohms from emitter to the NIM signal.Put 1Kohm from collector to +5v. Collector is your output.

The output TAUwill be the 1Kohm * Sum_of_capacitance on the collector, including Cob of the 2N3904. For more speed, reduce the 50 ohms and reduce the 1Kohm.

2) Use a high-speed TTL or CMOS inverter or Schmidt. Use 2 resistors to level translate from -.8/0 to +0.8/+1.4 volts, with a speedup cap across the bottom resistor. The "two resistors" could just be a cermet pot, value 10Kohm??

3) Use the AM685 (AMD used to sell it) comparator. Easily handles 100MHz sins.

\$\endgroup\$

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.