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Audio Amplifiers Forum
OPA1632 Unity Gain Stable?
I would like to use the OPA1632 in a balanced audio driver application. I would like to know if this device is unity gain stable, and what the maximum allowed load capacitance is. As this will be driving long audio cables, I need to know what the limitations are. Alternatively, is there a different fully differential amplifier that you can suggest? Thanx. Kevin
Did you look at the DRV134?
Audio Applications Engineering Manager
Dallas, TX USA
Hi Don. Thank you for your feedback. I have used the DRV134 before, really is a great device, but only has single ended input. I require a device that is balanced in and balanced out, so the OPA1632 is a good option. I am just concerned with driving long lenghts of cables with it, if it would remain stable like the DRV134 is. Looking forward to your feedback. Thanx
The OPA1632 datasheet has a bandwidth line item spec for unity gain and a plot or two in the apps section showing unity gain performance. So the first answer is yes, it is unity gain stable.
As for Capacitive loading – this part is a high speed amplifier and is more sensitive to capacitive loading placed directly on the output. It can safely handle up to about 5pF to 10pF in unity gain configuration. But as this capacitance increases, more and more peaking will occur which can result in instability.
There are several options to consider. The most common is to place a series resistor on each output pin of the OPA1632. It is recommended to use at least 20-ohms palced very close (<10mm is ideal). There are other methods to isolate the capacitive loading such as a similar configuration as Fig 11 in the OPA1632 datasheet shows or even use a ferrite chip instead of a resistor with a low-Q and say 80 ohms @ 100MHz of impedance.
Looking at an alternative fully differential amp is a recommended route as a low power alternative would be helpful while still providing excellent audio performance with low quicescent current.
A low power alternative to the OPA1632 is the THS4521, although the THS4521 does not have quite as low a noise spec as the OPA1632 and the BW is slightly lower at 145MHz vs 180MHz.
I would be able to porvide more recommendations; could you provide me with the key specs you are looking to meet? (BW, Noise figures, THD, Distortion, etc.)
Also, do you have an ADC in mind already? We have a variety of ADCs that may be suitable for your design.
Ranji C. Bhola
Thank you for your comments so far.
This will not be used to drive an ADC directly. It will be used in an application where a balanced audio signal will be split before being distributed to two remote audio units, over balanced audio cable of about 100 metres. So first, since it is for analog audio application, the BW can be less. Also, will typically be used in a mode where gain =1. Requirements are for lowest noise possible, and needs to be stable with long lenghts of cable. (typical requirements for high audio fidelity applications).
Let me know if you need more info please. Thanx
I would recommend the THS4531, the BW is within the audio range, the min stable Acl gain (V/V) is 1, and the quiescent current is 0.25mA per channel which is a dramatic improvement over the OPA1632. This fully differential amplifier should fit your application needs.
There is a detailed section for Audio performance in the THS4531 datasheet including audio on/off pop performance, THD+N, and FFT test data of the device performance in the audio band.
Please let me know if you have further concerns.
Thank you very much for your assistance in this regard .I will have a look at the devices you suggested. Will let you know if I have any additional questions.
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