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AD1672 Datasheet(PDF) 14 Page - Analog Devices |
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AD1672 Datasheet(HTML) 14 Page - Analog Devices |
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14 / 20 page  AD1672 REV. 0 –14– GAIN AND OFFSET ADJUSTMENT The AD1672 is factory trimmed to minimize gain, offset and linearity errors. In some applications, the gain and offset errors need to be externally trimmed to zero. Since the gain and offset errors of the AD1672 are interdependent, an iterative process is required to trim both errors. The OTR pin can be monitored to trim the offset and gain errors to within 1/2 LSB of negative and positive full-scale (i.e., –FS and +FS) respectively. In this case, the calibration procedure would be to iterate between –FS and +FS voltage levels which are applied to the input of the AD1672 and adjust the offset and gain until OTR toggles at both –FS and +FS. Both gain and offset errors may be trimmed with external cali- bration circuits based on a potentiometer or a DAC. Digital calibration circuits using a DAC offers greater flexibility and can be programmed for various operating conditions. The fol- lowing digital calibration circuits are based on the DAC08 which is a low cost, 8-bit current output DAC with 85 ns set- tling time performance. Different adjustment spans and resolu- tion may be achieved by judicious selection of the resistors in the circuit. Figure 28 shows an offset correction circuit for the unipolar input ranges of 0 V to 2.5 V and 0 V to 5.0 V. The complemen- tary current outputs of the DAC08 provide a bipolar adjustment range at the output of the op amp if RS is made equal to RF/G. G is the noninverting gain of the op amp and is equal to 1 + RF/RA. REFOUT divided by RIN sets the full-scale current output, IFS, for the DAC08 Note, RP is a pull-up resistor used to source additional current if IFS is greater than 500 µA. The bipolar voltage adjustment span at the output of the op amp is equal to IFS × R F. RI isolates the op amp from the DAC08 output capacitance. Note, that the values of these resistors can be optimized for any circuit requirement or adjustment span since they are not uniquely defined for any given voltage span. RF RA RI RI VREF(+) VREF(–) DAC08 I O I O RIN RIN 1µF RP AD1672 AIN REFOUT +VCC Figure 28. Unipolar Offset Correction Using the DAC08 Figure 29 shows an offset correction circuit for the bipolar input range of –2.5 to +2.5 V. This circuit is similar to the circuit shown in Figure 28 except that the op amp is configured in an inverting topology so that the voltage compliance of the DAC08 is not exceeded. RF RA RI RI VREF(+) VREF(–) DAC08 I O I O RIN RIN 1µF RP +VCC RS AD1672 AIN REFOUT Figure 29. Bipolar Offset Correction Using the DAC08 Figure 30 shows a gain correction circuit which is also similar to the circuit shown in Figure 28. The circuit consists of a unity gain amplifier in which RS and RF are equal to 158 Ω. In this case, the full-scale output current of the DAC08, IFS, is set at 500 µA to minimize the load to the AD1672 REFOUT. The output of the op amp can be varied over a ± 78 mV range around the midscale voltage of REFOUT. Also, RS and RF are selected such that 1 LSB of the DAC08 corresponds to 1 LSB of the AD1672. 500 Ω 5k Ω 5k Ω 1µF 4.7k Ω +VCC AD1672 REFIN REFOUT 2.5V VREF(+) VREF(–) DAC08 I O I O 500 Ω 158 Ω 158 Ω 2.5 ±0.078V Figure 30. Gain Correction Using the DAC08 OUTPUT LATCHES The AD1672/EB evaluation board schematic in Figure 31 shows the AD1672 connected to the 74HC541 octal/buffer line drivers with three-state outputs. The latch can drive highly ca- pacitive loads (i.e., bus lines, I/O ports) while maintaining the data signal integrity. AD1672 EVALUATION BOARD GENERAL DESCRIPTION The AD1672/EB is an evaluation board for the AD1672 12-bit 3 MSPS analog-to-digital converter (ADC). Figure 31 shows the schematic for the AD1672/EB evaluation board. Careful attention to layout and circuit design combined with analog and digital proto- typing areas allows the user to easily and effectively evaluate the AD1672 in any application requiring high resolution, high speed conversion. The analog input to the AD1672/EB may be driven directly or via an onboard buffer amplifier. The AD1672/EB contains an 8-bit DAC which can be easily configured for input offset ad- justment or for reconstruction of the digital output. An optional external voltage reference is included for greater dc precision |
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Similar Description - AD1672 |
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