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  • Great item, but...

    posted by Sneuper on 04/22/2013
    8

    Involvement:Expert (understands the inner workings) - Ownership:1 week to 1 month

    Pros:

    : Great item, good value for your money !
    Complete with USB cable and test wires.

    Cons:

    Bad input circuitry behaviour when connecting to 5V logic levels. It takes a current of 7mA when connecting to a high 5V level ! Problem is type of inputbuffer used. No “5V tolerant inputs”, but can be modified.

    OtherThoughts:

    Inputbuffer is a 74HC245 chip powered with 3.3V coming from a 3.3V regulator. The buffer will provide some ESD protection for the inputs. (don’t know why a birectional buffer was chosen) The problem is the buffer being a HC type. When HC technology is powered from 3.3V you should not feed the inputs with a 5V signal! With HC technology, the maximum input signal level should not exceed Vcc + 0.5V .
    If you ignore this, the chip will take a lot of current and can break down. However, in this analyzer the current is limited because series resistors of 100 ohms were used. So the chip will NOT break down, but we can not call this “5V tolerant inputs” because they will be a heavy load for our connected DUT when signals rise above 3.3V. Note that the Cypress microcontroller ITSELF is 5V tolerant !
    So for a DUT with 3.3V logic levels the analyzer is O.K, for a DUT with 5V logic it will NOT offer high impedance inputs, which in fact is not a nice design.

    Bottomline:

    BUY IT if you don’t care to do some modifications!
    1. One can replace the 74HC245 chip with a 74LCX245 or equivalent, which is 5V tolerant with its Vcc = 3.3V This will be the best solution and the analyzer will have high impedance inputs for all signals from 0 to 5V. This modification calls for very good eyes and no shaking hands…
    2. One can modify the pcb tracks in such way that the 74HC245 will be powered from the 5V USB voltage. This will work O.K when connected to a 5V logic DUT, but now in contrast we can get in trouble when we connect a DUT with 3.3V logic: the level of the high signal from the 3.3V DUT is maybe not always high enough now for being a logic ‘1’ at the 74HC245 inputs…..
    Adding a select switch to choose the Vcc of the existing 74HC245 chip between 5V of the USB or 3.3V from the regulator could be an option. But the housing is very small and maybe you have to put the PCB in a larger box then.

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