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发表于 2013-7-16 19:52:05
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本帖最后由 shangdawei 于 2013-7-16 19:53 编辑
51kingst 发表于 2013-3-22 16:32 ![](static/image/common/back.gif)
他比较的方法,是用了DA输出的值,给后级电阻的网络一个REF的值,进来的信号加上这个REF的值,就是实际送 ...
http://openschemes.com/2010/03/2 ... e-the-modification/
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.
http://www.amobbs.com/forum.php? ... p;page=1#pid5531295
2、触发电压虽然可调,但由于没有采用比较器(高速比较器非常非常贵),而是采用了类似固纬的逻辑分析仪的输入电路,
输入电压如果接近设置电压时,可能会产生不定值(TTL、CMOS电平的定义决定这个问题)
具体如何根据选择的电压来调整VDAC呢 ?
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