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CA3338E Datasheet(PDF) 6 Page  Intersil Corporation 

6 page 6 CA3338, CA3338A In bipolar operation, VREF would be returned to a negative voltage (the maximum voltage rating to VDD must be observed). VEE, which supplies the gate potential for the output drivers, must be returned to a point at least as negative as VREF. Note that the maximum clocking speed decreases when the bipolar mode is used. Static Characteristics The ideal 8bit D/A would have an output equal to VREF with an input code of 00HEX (zero scale output), and an output equal to 255/256 of VREF+ (referred to VREF) with an input code of FFHEX (full scale output). The difference between the ideal and actual values of these two parameters are the OFFSET and GAIN errors, respectively; see Figure 3. If the code into an 8bit D/A is changed by 1 count, the output should change by 1/255 (full scale output  zero scale output). A deviation from this step size is a differential linearity error, see Figure 4. Note that the error is expressed in fractions of the ideal step size (usually called an LSB). Also note that if the () differential linearity error is less (in absolute numbers) than 1 LSB, the device is monotonic. (The output will always increase for increasing code or decrease for decreasing code). If the code into an 8bit D/A is at any value, say â€śNâ€ť, the output voltage should be N/255 of the full scale output (referred to the zero scale output). Any deviation from that output is an integral linearity error, usually expressed in LSBs. See Figure 4. Note that OFFSET and GAIN errors do not affect integral linearity, as the linearity is referenced to actual zero and full scale outputs, not ideal. Absolute accuracy would have to also take these errors into account. Dynamic Characteristics Keeping the fullscale range (VREF+  VREF) as high as possible gives the best linearity and lowest â€śglitchâ€ť energy (referred to 1V). This provides the best â€śPâ€ť and â€śNâ€ť channel gate drives (hence saturation resistance) and propagation delays. The VREF+ (and VREF if bipolar) terminal should be well bypassed as near the chip as possible. â€śGlitchâ€ť energy is defined as a spurious voltage that occurs as the output is changed from one voltage to another. In a binary input converter, it is usually highest at the most significant bit transition (7FHEX to 80HEX for an 8 bit device), and can be measured by displaying the output as the input code alternates around that point. The â€śglitchâ€ť energy is the area between the actual output display and an ideal one LSB step voltage (subtracting negative area from positive), at either the positive or negativegoing step. It is usually expressed in pV/s. The CA3338 uses a modified R2R ladder, where the 3 most significant bits drive a bar graph decoder and 7 equally weighted resistors. This makes the â€śglitchâ€ť energy at each 1/8 scale transition (1FHEX to 20HEX, 3FHEX to 40HEX, etc.) essentially equal, and far less than the MSB transition would otherwise display. For the purpose of comparison to other converters, the output should be resistively divided to 1V full scale. Figure 5 shows a typical hookup for checking â€śglitchâ€ť energy or settling time. The settling time of the A/D is mainly a function of the output resistance (approximately 160 â„¦ in parallel with the load resistance) and the load plus internal chip capacitance. Both â€śglitchâ€ť energy and settling time measurements require very good circuit and probe grounding: a probe tip connector such as Tektronix part number 131025800 is recommended. 255/256 254/256 253/256 3/256 2/256 1/256 0 00 01 02 03 FD FE FF = IDEAL TRANSFER CURVE = ACTUAL TRANSFER CURVE OFFSET ERROR (SHOWN +) GAIN ERROR (SHOWN ) INPUT CODE IN HEXADECIMAL (COMP = LOW) FIGURE 3. D/A OFFSET AND GAIN ERROR 0 00 C B FROM â€ś0â€ť SCALE INTEGRAL LINEARITY ERROR (SHOWN ) STRAIGHT LINE TO FULL SCALE VOLTAGE INPUT CODE = IDEAL TRANSFER CURVE = ACTUAL TRANSFER CURVE A = IDEAL STEP SIZE (1/255 OF FULL B  A = +DIFFERENTIAL LINEARITY ERROR C  A = DIFFERENTIAL LINEARITY ERROR A SCALE â€ś0â€ť SCALE VOLTAGE) FIGURE 4. D/A INTEGRAL AND DIFFERENTIAL LINEARITY ERROR 

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