r********g
发帖数: 868 | 1 用的是SN74LS221D, CLR Bar 接高, A 接地, B 进来一个不是很规则的脉宽约10-15ns
的3.3V脉冲, Rext和Cext之间用的是普通的陶瓷电容,不是用的推荐的有正负极的电容,
结果没有出来脉冲. 我觉得原因可能有两个: 第一个是B的脉宽太小,但用的是边缘触
发,应该没问题;另一个可能是必须要用正负极的电容, datasheet上有这么一句: due
to the internal circuit, the Rext/Cext terminal never is more positive than
the Cext terminal. 清大牛们帮忙分析下原因,谢谢 | f*****0
发帖数: 489 | 2
15ns
容,
than
it is too fast for the part.
polarity of the timing cap isn't an issue: you simply cannot find a 22pf
electronic or tantalum.
【在 r********g 的大作中提到】
: 用的是SN74LS221D, CLR Bar 接高, A 接地, B 进来一个不是很规则的脉宽约10-15ns
: 的3.3V脉冲, Rext和Cext之间用的是普通的陶瓷电容,不是用的推荐的有正负极的电容,
: 结果没有出来脉冲. 我觉得原因可能有两个: 第一个是B的脉宽太小,但用的是边缘触
: 发,应该没问题;另一个可能是必须要用正负极的电容, datasheet上有这么一句: due
: to the internal circuit, the Rext/Cext terminal never is more positive than
: the Cext terminal. 清大牛们帮忙分析下原因,谢谢
| r********g
发帖数: 868 | 3 有可以推荐的one shot么，或者怎么把短脉冲拓宽？谢谢
【在 f*****0 的大作中提到】
:
: 15ns
: 容,
: than
: it is too fast for the part.
: polarity of the timing cap isn't an issue: you simply cannot find a 22pf
: electronic or tantalum.
| f*****0
发帖数: 489 | 4
right off my head, no.
But I would try some high speed versions of the same chip (HCT or ESL
for example). or you may want to try the low voltage version (LV
version, it takes less time to switch at lower voltage, everything else
is the same). you may also try to run the CMOS version (CD) at higher
than spec voltage (fet's Ciss goes down with Vgs, making them faster).
or try different manufacturers to see if they have faster versions of
the chip.
【在 r********g 的大作中提到】
: 有可以推荐的one shot么，或者怎么把短脉冲拓宽？谢谢
| r********g
发帖数: 868 | 5 非常感谢，如果想拓宽初始脉冲，怎么做？用opamp?
【在 f*****0 的大作中提到】
:
: right off my head, no.
: But I would try some high speed versions of the same chip (HCT or ESL
: for example). or you may want to try the low voltage version (LV
: version, it takes less time to switch at lower voltage, everything else
: is the same). you may also try to run the CMOS version (CD) at higher
: than spec voltage (fet's Ciss goes down with Vgs, making them faster).
: or try different manufacturers to see if they have faster versions of
: the chip.
| f*****0
发帖数: 489 | 6
chips like 221 are supposed to do exactly like that.
in general, analog circuits (like an opamp) isn't best suited for such
high speed / high slew rate applications, especially VFB opamps. If you
really want to do it with an opamp, you can do some research and my
guess is that your best chances are with CFB opamp which tend to have >
100mhz bandwidth and slew rates over 1000 v/us. to achieve something
like that with a VFB opamp, you likely will have to rely on positive
feedback and that may in
【在 r********g 的大作中提到】
: 非常感谢，如果想拓宽初始脉冲，怎么做？用opamp?
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