Build Details III – Trigger Buffers
You’re almost there! The next circuit that needs to be installed is the op-amp trigger buffers located at the top of the board around the LM348. Most likely, two channels (A & B) are already installed so you will only have to solder a few more components in order to complete the hardware.
We’ll start out with the schematic and description of the buffer circuit. The Zeroplus DV4128B that is located next to the LM348 is a serial DAC used to provide a variable voltage to the noninverting terminal of each of the 4 op-amps in the ’348.
Fig 13 – Schematic of the Trigger Buffer (each channel)
The op-amp will drive it’s output to whatever is needed in order to make the inverting terminal match that DAC voltage. Since the inverting terminal is connected to a “1.2v reference”, the difference between that DAC voltage and 1.2v will cause current to flow through the 10k resistor. And since that current must flow from the op-amp output through the 36k resistor, the output of the op amp will be VDAC + 3.6(VDAC-1.2v). The 3.6 comes from the ratio between the 10k and the 36k. In this manner, the output of the op amp can be adjusted both above and below 1.2.
This variable output voltage is applied to the top of all the 3x 1MEG resistor dividers in the input circuits for this channel. In this way, the trip point of the input voltage (trigger threshold) can be varied. The ’245 always has a fixed input trip point, but the variable buffer voltage and the input circuit adjusts various input voltage ranges to always match the ’245′s trip point. The software allows the user to choose TTL, CMOS, ECL, or user-defined levels for the trip point. It is these selections that determine the DAC and buffer output voltage.
In the interest of completeness, the component layout for all 4 channels of the buffer circuit is shown below. As was mentioned prior, you will probably only be installing 2 channels, so some of these components will already be placed – there is no need to move or adjust those devices, obviously.
Fig 14 – Component Layout of the Buffer Circuit
In addition to the missing components around the LM348, there are usually two missing capacitors over by the DV4128 DAC. These are both 120nF capacitors – probably overkill as we are placing parallel VDAC caps near the LM348, but we went ahead and installed them anyway.
Here’s a pic of our completed buffer circuit. As you may remember, we lost our 1.2uH inductors or may have left them at the store! So we installed jumper wires where the 1.2uH inductors would go. These small wires have SOME inductance, probably 150nH, but not the right value. We’ll let you know if skipping these components causes us any problems in the future. Our thinking was: If digital noise freaks out the LM348, we’ll go get more inductors and install them. If it kills the ’348, it’s cheap to replace. So our laziness prevails and in the end we found it works fine. But our recommendation to you is to install all the proper components unless you’re willing to debug when something goes wrong.
Fig 15 – Final pic of the LAP-C rebuild. A little messy, but works just fine.
You may be wondering about the little white wire jumper on buffer A. Funny story – the pin next to the noninverting terminal on op-amp A is the negative supply voltage, -12v. During a probing session, the meter probe slipped off the + terminal and touched between the + terminal and -12v. This killed channel A of the DAC – blew it clean out and channel A of the DAC just floats dead now. It’s amazing that the failure mode is a floating open, as we would have guessed that pulling a 3.3v (probably 0.35um) output to -12v would have latched up the entire DAC chip and turned it into a smoking crater!
The op-amp survived the event unharmed, so we tied his + terminal to the quasi-1.2v reference, and port A is now hardwired to a 1.2v (TTL) trigger voltage. That’s fine for us and we are still thanking our lucky stars that ZeroPlus used some kind of structure OTHER than a CMOS push-pull for their DAC output, or we would have blown the entire DAC. The moral of the story is: Be careful when probing a live circuit, folks. And the second moral is: If you blow a DAC output, you can always just hardwire the trigger voltage.
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