TI高精度实验室-压摆率 1.pdf.pdfTI高精度实验室-压摆率 1.pdfpdf,TI高精度实

文件名称: TI高精度实验室-压摆率 1.pdf.pdf

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详细说明：TI高精度实验室-压摆率 1.pdfpdf,TI高精度实验室-压摆率 1.pdf压摆率主要描述∫运放在大信号输入时的响应指标,而在正负100mV以下的小信号输入时,我们应该使用小信号带宽这一指标,二者是不同的。 Time Capacitor Voltage voltage change Time Potentiometer wiper moving slowly"up Capacitor current Time EXAS INSTRUMENTS o before we get into an in-depth slew rate discussion, let s first review some basics tion that defines how a capacitor works states that the current flow through a capacitor is equal to the capacitance times the derivative of voltage with respect to time. this behavior can also be interpreted to mean that if you have a constant current then the voltage across the capacitor will rise linearl over time ●在深入讨论压摆率之前,让我们先来复习一些基础知识。 ●这个等式描述的是:流经电容器的电流等于电容器容值乘以电容器两端电压随时间的变化率。当电流恒定时,电容器两端电压将会随着时间成线性变化从而可以表示为v=mt.其中,v(t)是电压的瞬时值,m是图中直线的斜率 TEXAS INSTRUMENTS o This is important with respect to slew rate of an amplifier. An amplifier has an internal gM, or transconductance, stage which takes the input differential voltage and converts it to an output current, loUT. IOUT flows into the next stage where it is used to charge CC, which is called the Miller capacitance. If IOUt is a constant, then the voltage across cc will rise linearly with time, just like we discussed on the previous slide o For slow-moving signals, IOUT is less than some maximum value IOUT_ MAX. This means that IOUT is able to change according to the differential input voltage without being limited But for rapidly moving, large signals, IOUT reaches its maximum and becomes limited to some constant value. In this case the input to the amplifier will no longer be a virtual short and therefore a differential voltage will develop across the input pins Since loUt is constant voUt across the miller capacitor cc increases linearly over time. This is when the output of the amplifier is considered to be slew rate- limited, which is fastest that the output voltage can change ●压摆率是运放的一个很重要的参数。下面我们等效画出运放的输入级和放大级。输入级有个跨导增益gm,它把运放差分对管输入的电压转化为本级输出电流,lout。lout流入放大级,并对放大级的密勒电容,即图中Cc进行充电,根据上一页幻灯片的说明,当lout是常数时,cc两端电压将会线性增加。对于缓慢变化的信号,lout远小于本级的饱和输出电沇 loutmax,这说明lout 会随着输入差分电压变化。但对于快速变化的大信号,Jout将会达到其饱和电流值。在这个例子中,lout饱和后,运放的输入将不再是虚短路,即运放的正负输入端引脚电压不再相等。因为lout达到饱和成为常数,Cc两端的电压Vout将会随时间以固定斜率线性增加,此时就认为运放达到压摆极限, 即其输出转换速度达到了最快。 TEXAS INSTRUMENTS o Here is a transistor-level view of what s happening inside the amplifier. When we apply a step input to the amplifier, which is an extremely fast-moving signal one transistor in the gM stage will be turned off and the other will be turned fully on. The current flowing through the transistor which is ON, is the lOUT MAX mentioned in the previous slide as previously discussed IOUT MAX flows into the Miller capacitance CC, causing the output voltage to ramp linearly over time 接着上一页pot的说明,这里以三极管输入的运放为例。当我们在输入的差分对管上施加一个快速变化的大阶跃信号时,有一个三极管将会截止,另一个将公饱和。这时流经饱和三极管的电流就是上一页幻灯片中提到的 loutmax。像前面讨论的一样, outman流经αc后,将使out线性增加。 TEXAS INSTRUMENTS o Here we compare the typical slew rate and quiescent current, or IQ, for different amplIfiers o On one end of the spectrum, we have the OPA3 69 which is a very low IQ and low slew rate device. For 0.8ua of current we can achieve around 5mv/us of slew Compare that to the opa847, which consumes 18 1mA of iQ but can slew at 850V/us. This shows us that amplifiers with higher slew rate, and therefore higher bandwidth, tend to have higher current consumption ●这里我们比较些运放的压摆率和静态电流l之间的关系,比较中采用的参数均是其典型值。 ●在表中第一行中,可见OPA369的静态电流非常低,为0.86uA其压摆率为 5mv/us,与最后一行静态电流181mA的OPA847相比,OPA369的压摆率远小于OPA847的850V/us。这说明运放的压摆率越高,带宽越大,其消耗的电流也越大。 SR÷899mV-(-904my=005 7.43-5.16u SR=0.8 From data sheet 且幽易四组、O限果卖加N地 y:399725m 16u xC.2≤7u y:180A R1 Ink -90563m 11. ctu 2C.00u TR eula H TEXAS INSTRUMENTS o We can easily simulate slew rate using TINA-Tl. Simply apply a step function to the input of the amplifier which in this case is a +1v square wave you can see that when this input step is applied the input offset voltage changes from oV which indicates a virtual short -to some other voltage, around 900mv in this case. Most importantly, the output voltage becomes slew rate-limited, shown as a constant ramp in voltage over time until finally reaching its true value. You can observe the input offset voltage moving linearly back to ov as well o Calculating the slew rate from this plot gives a result of 0.795V/us. The data sheet for this device, the OPA2188, lists the slew rate as 0.8V/us, indicating that the model accurately simulates the slew rate of the amplifier ●我们可以使用τina来很轻松地仿真压摆率。使用τina里的信号源,给运放 OPA2188输入端加入一个幅值为正负1V的方波信号,在仿真结果中,可以看到在阶跃处,输入失调电压从虚短路时的0ν变化到了90mV附近。更重要的是,输出电压达刭∫压摆上限,即输出不像输入一样按方波变化,而是在输入阶跃发生处,输出按固定斜率上升或下降,逐渐到达正确的输出结果并稳定下来,而此时输入端的失调也线性地逐渐减小到0. ●从仿真可以计算得到运放OPA2188的压摆率为0795V/us,十分接近数据手册给出的0.8v/us,这同时也说明OPA2188的 spIce模型在压摆率方面可以准确的模拟真实器件。 Noname-TR result2 16u 04.24m8 x227u y1804 CC5 63m 281 U2 0P-2109 TEXAS INSTRUMENTS o This slide emphasizes that fact that we no longer have a virtual short whenever a step function is applied to the input of the amplifier the output moves slower than the input signal, and so we have some finite voltage across the input pins As the output ramps linearly to its final value, the input gets closer and closer to a virtual short again, and once it does the amplifier returns to its closed-loop conTiguration ●本页幻灯片着重说明当运放输岀信号以压摆率变化时,运放输入端不再满足虚短路的特性。因为输出变化比输入变化率,所以输出反馈到输入后,可以在运放正负输入端看到压差,随眷输岀电压逐渐线性的到达其最终值,输入引脚间电压逐渐减小,最终运放输入重新满足虚短路特性。

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