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DSP202 Datasheet(PDF) 10 Page - Burr-Brown (TI) |
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DSP202 Datasheet(HTML) 10 Page - Burr-Brown (TI) |
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10 / 19 page  ® DSP201/202 10 critical internal grounds, and care should be taken especially at these points to make them as close as possible to the same potential as the system analog ground. The design of the DSP201 and DSP202 insures that these pins will have minimal current flowing through them. Internally, power currents are directed to the digital grounds (pins 18, 19, and 27) for internal digital currents, which are primarily switching currents, and to the analog grounds (pin 28, plus pin 4 on the DSP201) for analog currents, which are primarily from the internal current switches and the output amplifier. Pin 16 on the DSP201 is used internally as a logic level, and injects essentially no current into the ground. Wherever possible, it is strongly recommended that separate analog and digital ground planes be used. With an LSB level of 92 µV in 16-bit modes, and one quarter of that in 18-bit modes, the currents switched in a typical DSP system (processor, memory, etc.) can easily corrupt the output accuracy of the D/A’s unless great care is taken to analyze and design for current flows. POWER SUPPLY DECOUPLING All of the supplies should be decoupled to the appropriate grounds using tantalum capacitors in parallel with ceramic capacitors, as shown in Figures 2 and 3. For optimum performance of any high resolution D/A, all of the supplies need to be as clean as possible. If separate digital and analog supplies are available in a system, care should be taken to insure that the difference between the analog and the digital supplies is not more than 0.5V for more than a few hundred milliseconds, as may occur at power-on. Separate –5V analog and digital supplies are not needed. These pins are kept separate internally to minimize cou- pling. Drive pin 20 from the –5V analog supply, and make sure that the decoupling shown in Figure 2 or 3 are placed as close as possible to the D/As. CALIBRATION AND ADJUSTMENT OPTIONAL EXTERNAL OFFSET AND MSB TRIMS All of the specifications for the DSP201 and DSP202, plus the typical performance curves, are based on the perfor- mance of these D/As without external trims. In most appli- cations, external trims are not required. If external trims are not used, pins 23, 24, and 25 on the DSP201 should be left open, as should pins 2, 3, 23, 24 and 25 on the DSP202. These pins should not be decoupled with capacitors or tied to any specific potential, or the noise on the D/A outputs may increase. ADJUSTING OFFSET Where required by specific applications, offsets can be trimmed using the circuits in Figure 2 (DSP201) or Figure 3 (DSP202.) As with all standard D/As, offset on the DSP201 and DSP202 means the difference of the output from the ideal negative full scale value. The DSP201 and DSP202 use a current switching D/A architecture, and the current from this is internally amplified to produce a ±3V output range. Negative full scale output thus results from having all of the internal current switches turned off. Offset on the DSP201 and DSP202 should not be confused with the delta from 0V with an input code of 0000...0000 (0000 hex for 16-bit Modes, 00000 hex for 18-bit Modes). This is often described as bipolar zero error, and includes the effects of both offset and gain error. To trim the offsets, first latch the D/As with 1000...0000 (8000 hex or 20000 hex). Then adjust the offset adjustment pots to produce an output of –3.000000V. ADJUSTING THE MSB WEIGHT The MSB adjustment circuitry shown in Figure 2 for the DSP201 and in Figure 4 for the DSP202 basically change the weight of the MSB by adding to or subtracting from the current controlled by the internal MSB switch. Depending on the application, the MSB adjustments can be made in one of three different ways to optimize the system performance using the DSP201 or DSP202. For dynamic performance, the MSB can be adjusted to minimize distor- tion of either a full-scale or low level sine-wave output. For applications stressing differential linearity, the 0000...0000 (0000 hex or 00000 hex) to 1000...0000 (FFFF hex or 3FFFF hex) transition can be trimmed to change the output of the D/As precisely 1 LSB (92 µV in the 16-bit Mode or 23µV in the 18-bit Mode.) To adjust for minimum distortion of full-scale sinewaves, strobe the inputs to the DSP201 or DSP202 with codes representing ideal full scale sine waves, then trim the MSB adjustment circuit to minimize distortion, as measured by either a distortion analyzer or by digitizing the output with an appropriate A/D and running FFT analyses. In many audio applications, it is more appropriate to adjust for minimum distortion with low level sinewave outputs. This minimizes zero-crossover error, which can be a con- cern in high-end audio systems. To do this, strobe the inputs to the DSP201 or DSP202 with codes representing ideal low-level sine waves (–60dB from full scale works well), and then trim the MSB adjustment circuit to minimize distortion, again using a distortion analyzer or FFT analyses to check the results of the trims. The MSB adjustment circuits can also be used to trim the D/A outputs directly for the transition from 0000...0000 (0000 hex or 00000 hex) to 1111...1111 (FFFF hex or 3FFFF hex), eliminating differential linearity error at the major carry. Ideally, this transition of the digital input code should cause the D/A outputs to change 92 µV in the 16-bit Mode or 23µV in the 18-bit Mode. A simple way to make this adjustment is to continually load alternately the codes 1111...1111 (FFFF hex or 3FFFF hex) and 0000...0000 (0000 hex or 00000 hex) into the DSP201 or DSP202. An amplifier with sufficient gain can then drive an oscilloscope input, and the transition output step can be adjusted. |
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