I'm looking for a low noise opamp that can operate with a negative supply rail of 0 and a positive supply rail of 20V. The AD797 specifies absolute maximum limits of +/- 18V. Will I be safe to use the AD797 with my 20V range as described?
From a recent email to another customer:
“ We get two questions a lot:--- Your op amp is spec'ed on +/V, will it work single supply?? OR--- Your op amp is spec'ed on ground and +V, can I operate it on +/- supplies? With respect to dual supplies vs. single supplies, ALL op amps can run on both.The little electrons inside the op amp don't know where ground is, becausethere are no ground pins on modern op amps.So an op amp that is spec'ed for +/-15V operation, can run on +100V and+130V PROVIDED you observe the input voltage range (IVR) and outputswings. The first op amps did not have RR inputs or RR outputs, so whenNational introduced the LM324 in 1975, it had a PNP input stage, so the IVR (Input Voltage Range) included the V- pin. So you could run it onground and +V. So the official definition of single supply is:"An op amp whose input range includes the V- pin".To stay with this example, can I say to a customer, "Yes, you can use thepart at dual supply +100V/+130V ? Does the part work in this case likespecified at +-15V? Do we guarantee this?"All specification data in the +-15V table, exclusive of input/outputrange are valid; in other words, CMRR, PSRR, Isy, etc. are the same.Although note that the CMRR test conditions might be -10V <Vin < +10V, soyou would have to adjust for this.When I was talking my network analysis course, the professor said you couldpick any node for ground and write your equations for the other nodes in termsof that reference point. He also said to pick a direction for current through a resistor. If the numbercame out negative, it was going in the other direction.Again, there is no ground pin on the op amp, so if we measure a Vos of100 uV on an op amp with +/-15V supplies, and then move everything in the testcircuit up by 115V, then you will have 100V on the V- pin and 130V on the V+ pin. But the silicon doesn't know this,so the Vos is still 100uV.You could operate any op amp that is specified at +/-15V at 100V/130V with oneBIG caveat: You have to keep the inputs and outputwithin the rails. If you have an op amp that is specified at +/-15V and the IVR is -11V to +11V,then on 100/130V, the IVR would be 104V to 126V.In real life, this is not practical, because of power up/power down.Let's say you have a cap on the output of the op amp, and it is charged to+110V. When you shut off the power and the V- pin and the V+ pin go to zero, the cap will discharge back into the output stage and blow up the part."
So in short, running on +20V and ground is not a problem, provided you don'thave any spikes on power on overshoot.
The IVR is +/-121, so that translates to +4V to 16V for operation on groundand +20V.
As a side note, if you are going to build this for awhile, I would use the
next gen part, the AD8597, a much newer part that should be around longer.
I am designing a new PLL system based on HMC699 PLL which needs to use an active filter topology. From the ADISim PLL simulator I have used AD797 as operational amplifier and the simulations went ok with power supply from +12VDC to GND.
As you say in the previous response it is possible to apply +20V to GND power supply but I am not sure if I have understand this :
You could operate any op amp that is specified at +/-15V at 100V/130V with oneBIG caveat: You have to keep the inputs and outputwithin the rails.
Which is the IVR of tjhe AD797 whith +20 to GND power supply?
If the IVR stands for INPUT COMMON-MODE VOLTAGE RANGE, it seems that 12 VDC to GND could not be the best option. Is this wrong?
Thanks in advance
Allow me to respond regarding your concern.
+ 12 V corresponds to an IVR when operating the device at + 15 V supply. Using this information, we can deduce that the amplifier can accept an input voltage signal of 3V from the rails before clipping. For 12 VDC to ground, the IVR will be +3 V to +9 V.
Depending on your filter requirements, you can also try the latest ADA4625-1 18MHz GBP, which is perfect for PLL applications. For your +20V (pos supply) and 0V (neg supply), ADA4625-1 will have IVR of +16.5 to 0V.
Thanks for your response, my power supply is +12V to 0V, in previous responses I have been told that I will get an IVR from +3V to +9V. Would It be correct to use AD797 or shoul I use another part such as the one you recommend, taking into account that I only have +20V.
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