OK. Slew Rate = 2 x π x 20,000 (Hz) x 28 volts (peak) = 3,518,583.772 times 4 for safety = 14,074,335.09
Huh?
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You've quoted a peak value of 28V which is actually the peak to peak. So your final number is too high by a factor of two. Actually it isn't even that. The OP1656 will swing within 0.25V of the rails, so for +/-18V it will do 17.75V. So your equation should give 8.5V/us even with a factor of 4 for safety.
As compared with the OPA1656 which is 24V/us.
The question is rather moot anyway. There is no practical audio program material which will have 24dBu (which is what 17.75V peak corresponds to) at 20kHz.
The applications that slew rate limiting is important to take into account is using the device in non-audio applications.
Craig
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Huh? 100 watts = 20 volts rms @ 4 Ω. 20 volts rms = 28.18 volts PEAK.
Not peak-to-peak. That is for a POWER amplifier. Not OPA1656. 8.9 v/µSec for a OPA1656 at clipping(17.75 volts).
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Very nice info, thanks john! So having all that said, what's the best suited TI opamp for an I/V then? ANd what about the fully differential ones, what are the implications of using these for a differential I/V?
Thx you John , it makes a lot of sense ..
I was using a bipolar opamp with good slew rate and linear input stage... I assumed that I heard the better slew rate , now I understand that it was the linear input stage. (they are advertised as having linear input stage up to a few mV)
TI has OPA1656 and OPA2156 jfets that will work nicely for IV stage (while having low voltage noise)
I was using a bipolar opamp with good slew rate and linear input stage... I assumed that I heard the better slew rate , now I understand that it was the linear input stage. (they are advertised as having linear input stage up to a few mV)
TI has OPA1656 and OPA2156 jfets that will work nicely for IV stage (while having low voltage noise)
I always try to maximize the gain in the I/V stage of a DAC output. This is because the signal gain is directly related to the value of the feedback resistor, but the noise in the broadband region is usually determined by the feedback resistors thermal noise, proportional to its square root. That means there is an SNR benefit to maximizing gain. Plus its the first stage in the circuit, and you should maximize gain at this point anymore. So how do we pick an op amp?
You're looking for an op amp with a combination of voltage and current noise that provides the best noise performance at that particular source impedance (parallel combination of the DAC output impedance and the feedback resistor). If that impedance is <1kOhm, I would look at BJT input devices. >10kOhm? No brainer there, JFET and CMOS types. What about the murky world of source impedances between 1-10kOhm where it seems like every audio circuit ends up?! Here there are a number of options: BJT-inputs without bias current cancellation (OPA1622), Super-Beta BJTs (OPA2210), Low voltage noise JFET and CMOS (OPA1642, OPA827, OPA828, OPA1656/2156).
Of course there are a number of other factors to consider, like the ability to drive the input impedance of the following circuitry with low distortion levels. But I personally start with noise. Although I have seen plenty of debates on here about whether distortion sounds bad or good, I have never seen anyone argue that noise sounds good.
On the topic of fully differential amplifiers (FDAs): while at first glance this might seem like a great idea, it's important to note that the common mode voltage at an FDA's inputs is a function of the output common mode voltage, and the input signal. Usually current output DAC performance depends heavily on maintaining the proper voltage at the DAC output and that is not as straightforward on an FDA as it is with an op amp I/V stage. Its certainly something that a number of people have explored though.
OK, by maximizing gain you mean to increase the I/V resistor as far as possible, i.e. to achieve as big an output voltage one can handle in the following stages?
Ok, given the example I have at hand, the ES9038Q2M in mono mode: Output resistance of the DAC itself with both outputs parallelled is around 400Ohms which clearly falls into category one. BJT it is then. Any recommendations on this front? I guess the widely used OPA161(1|2) would be one of them?
This topic is a bit wierd regarding ESS DACs. Most peoply seem to adjust the voltage at the noninverting terminal of the I/V opamp so that it will output 0V in reference to GND. This will lead to a DC voltage of less than 1/2 AVCC at the output pins of the DAC, which in turn doesn't seem to bother it at all. Will have to test this. Or are you not talking about DC here? What's your opinion on wether this should make any noticeable difference at all, having the I/V outputs sitting at 0V or higher - with parts that have such little distortion the DC operating point shouldn't really matter, should it?
Stuart Yaniger (SY here) and I have visited Johnc124 last Friday in Tucson. In all the discussions it became clear that they have a lot of great audio parts coming out. Won't steal his thunder, but John clearly has diyaudio at heart. You will be pleasantly surprised.
Also received a small parting gift of about 50,000 OPA1656's. Each ;-)
Jan
Also received a small parting gift of about 50,000 OPA1656's. Each ;-)
Jan
Holy moly, so are we gonna see massive parallel OPA1656 version of Doug Selfs NE5532 Power Amp soon?
It was great having you guys visit! You'll have to come back for another tour when our new facility is finished.
Yes, indeed---all very good opamps!! BUT...the almighty $$$$$$:
OPA1622 $5.73 OPA2210 $5.17 OPA1642 $2.08 OPA827 $10.80 OPA828 $7.54 OPA1656 $2.72 OPA2156 $2.91
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That's with the aim of performance. On the other hand you can go to Analog Devices and get much more expensive opamps with about as good or worse performance.
If price is your concern, get some 5534 or perhaps njm8068 njm4585.
Also pricing for 1000+ isn't really that expensive for opa1656, opa1602, opa1612.