[EMAIL PROTECTED] wrote:
Hi Bruce,
would you have some sample schematics for a more modern, faster version of
this than let's say the one used in the 5334A etc that you could share?
thanks,
Said
Said
The Attached GIF file illustrates one possible approach to a TAC using
discrete components.
The opamp is a high speed op amp with a low bias current. The reset
switch Q4 is normally on as is Q2.
Just before the beginning of the pulse width to be measured Q4 is turned
off, then after a short fixed delay Q1 is turned on until the end of the
pulse.
The following ADC which has been tracking the output of the opamp then
begins a conversion at the end of which Q4 is turned on resetting the
integrator.
Connecting Q2 collector to the output of the opamp reduces the transient
current load seen by the opamp output stage.
The linearity is affected by:
1) The voltage dependence of the drain source capacitance of Q4 has to
be reduced, replacing Q4 a T-switch configuration will significantly
reduce this effect. The
2) The Early effect will change the effective turnoff switch threshold
of the longtailed pair. Fast transition times on the input signals at
the bases of Q1 And Q2 will reduce this effect.
3) The opamp input bias current will contribute some error, however this
effect is alleviated by always starting the ADC sample to hold
transition a fixed time after Q4 turns off.
4) The finite gain of the opamp will contribute some error, this can be
reduced significantly by using a composite amplifier using a pair of opamps.
5) The drift in the transistor common base current gain with temperature
will affect the TAC gain TC.
There are also a host of other small effects (e.g. dielectric
absorption) that contribute to TAC nonlinearity and tempco.
Bruce
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