we want use DAC61416 chip to get square wave voltage under the control of PWM (into toggle pin), but a big overshoot happen as the picture show. we test the output signal close to the chip pin, also install 51ohm or 100Kohm, this phenomenon do not disappear or change obviously. could you help check and solve this problem? we hope the signal is very accurate and smooth, so the overshoot is unacceptable.
we set the range of output voltage as +/-10V, actual value is +/-1V to +/-5V. At present, we do not add cap on the output.
next picture is the overshoot(60Hz)
by the way, we found pwm for toggle pin and output voltage have overshoot at the same time, but pwm signal is just for control, next is for 60Hz pwm and output. desired voltage is 60Hz +/-1.355V, actual value seems ok except the overshoot.
but after changing the frequency to 30KHz, pwm and voltage output do not have overshoot, but actual value is higher,up to about +/-1.57V, far away +/-1.355V, even though it seems close to square wave
we also attach the schematic and layout...
Please help provide your suggestion, we need accurate voltage, so the overshoot is unacceptacle.
Thanks a lot in advance.
Hi Akhilesh
we connector them to a connector and the connector should connector with our device. At present, we do not connector our device. The signal are testing close to DAC chip pin or connector.
What is more important, we found that if we want to generate square wave voltage, RC have to increase to reduce the overshoot, Good RC value can remove the overshoot. But the output quantities reach 50, total RC count will reach 100.
By the way, we also try to use Trigger Register value to replace PWM, but the overshoot still exist.
So could you help test your EVM board, to check whether it will also have overshoot when generating square wave signal? Or what should we do to reach no overshoot without adding RC? Please help provide your suggestion.
Does your output require current drive capability? If you did not have the RC filter, what would the load of the output look like?
I ask because we have a device called the DAC60096, which features 96 channels that operate at ±10.5V. This device has a toggling feature to generate square waves as well. The difference is that this is an unbuffered DAC, so you would not see any overshoot behavior because the DAC is just a resistor ladder. The problem with this is that current loads will create a voltage drop on the DAC. Capacitive loads will also slow the switching times as you create an RC with the resistor ladder and this load.
In regard to the DAC81416 family, consider that the output is an op amp. Op amps will overshoot based on their capacitive and inductive load. If this was an external amplifier, you could add compensation caps in the feedback loop to adjust the phase of the amplifier to achieve a balance between overshoot and settling time. The DAC81416 does not allow you to put a compensation capacitor, but the DAC81404 does allow this.
You can adjust the CCOMPx value until you get a good balance.
Using an RC filter with such a big cap is not a good solution as you have found. You would be better off using a large resistor and a small cap. You are forcing the device to continuously charge and discharge that cap, likely driving the output to its short circuit current limit. This high demand is likely what damaged your power supply. If you still want to try this, I recommend using 1nF only, but then change the resistor value.
Thanks,
Paul
Hi
current is not important, we just care the voltage. next picture show our load.
Cm is fixed,about 4pF, R011-R019(for example 0.3-15K).R101-R109(16K-60K),R501-R509(70K-200K), R011-R509 is more and more big.
if we do not add RC,direct output have big overshoot.
after adding RC(R=33ohm, C=1uF), the overshoot reduce a lot, but bring big transient current. we can not change cap to 1nF, because R have to increase a lot, it will affect the load. load resistant is variational.
do you have more suggestion about the chip?
