OPA549: High current, precision amplifiers for 18bit or 20bit DAC buffering

Hello Byliu,

I have been considering your very high resolution DAC/op amp application. The OPA549 is capable of approximately a +/-25 V output voltage swing when sinking/sourcing +/-8 Amperes out put current, with +/-Vs = 30 V. If an 18-bit DAC is considered it has 2e18, or 262,144 output codes. For a 20-bit DAC it has 2e20 or 1,048,576 output codes. If the full output range of 25 V peak is used the LSB voltage levels are about 95 uV for the 18-bit DAC, and 23.8 uV for the 20-bit DAC. Those are very small voltage levels relative to the OPA549 voltage offset, offset drift and noise voltages. For Example, the OPA549 voltage offset drift is typically ±20 µV/°C. If a large DAC code change takes place and the OPA549 is suddenly sourcing or sinking a high level of current the internal junction temperatures will rise very quickly causing the internal voltage offset to shift substantially. It can be seen that with a ±20 µV/°C drift that many bits of error can be easily had due to the self heating. Therefore, it isn't likely that you would be able to maintain 18, or 20-bit accuracy just due to offset drift alone. Then there is the Input Voltage Noise Density, en at f = 1kHz, of 70 nV/√Hz. That is a moderate noise level which was characterized under low power conditions. The noise of the op amp circuit would have to be analyzed to see if it would be low enough to support 18, or 20-bit performance.

I think in this case that a discrete high performance op amp, and an external complementary power transistor output stage could come closer to providing the high-level of electrical performance that your application requires. A low offset, drift and noise op precision op amp would be selected to drive the external NMOS/PMOS output stage. Such and op amp might be the OPA2192. The op amp could remain in a different thermal environment that the power output stage preventing it from undergoing large temperature swings. Certainly, the circuit design would have require much consideration to achieve 18, or 20-bit performance.

Please note that TI's Hi-Rel products and e2e are supported by TI's Hi-Rel products organization and they would be able to best assist you will product choices.

Regards, Thomas
Precision Amplifiers Applications Engineering

Hello Byliu,

Conceptually, this is the type of circuit you would look for using either bipolar power transistors, or power MOSFET devices:

 

Google search on a term such as "External Output Transistors Boost Output Current" or "Op amp output buffer circuit."

Here is an older TI/NSC Applications Report on the subject. The general information is still applicable:

http://www.ti.com/lit/an/snoa600b/snoa600b.pdf

I hope this helps get you started.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hello Byliu,

The dead zone you are referring to is associated with the class AB biasing of the complementary NPN/PNP output stage. Class AB reduces the output stage operating current when the output isn't delivering output current. That biasing increases the efficiency of the stage and reduces power dissipation. Increasing the idle current reduces the crossover distortion, but increases the power dissipation.

Fortunately, the effect of crossover distortion is reduced by negative feedback. The naturally occurring crossover distortion is divided by, and reduced by, the available loop gain; the difference between the open-loop gain (Aol) and closed-loop gain (Acl). Therefore, operating the op amp/buffer output stage with the lowest closed-loop gain, maximizes the loop-gain and minimizes the crossover distortion. It probably isn't apparent to most op amp users but in most cases the internal output stage in their favorite op amps is biased class AB as well. That goes for TI's high-performance audio op amps that produce THD + N, Total harmonic distortion + noise numbers on the order of 0.00005% (OPA1652). Therefore, class AB biasing is a good compromise, between distortion and power dissipation.

Please understand that achieving the 18 to 20 bit performance you are attempting is going to be challenging. There are things that may arise that haven't considered at this point, but all you can do is move forward and knock them down one at a time. Using a very high performance op amp such as the OPA211 gets you off to a good start.

Regards, Thomas

Precision Amplifiers Applications Engineering