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LTC1605 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1605 Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page 9 LTC1605 APPLICATIONS INFORMATION Internal Voltage Reference The LTC1605 has an on-chip, temperature compensated, curvature corrected, bandgap reference, which is factory trimmed to 2.50V. The full-scale range of the ADC is equal to ( ±4 • VREF) or nominally ±10V. The output of the reference is connected to the input of a unity-gain buffer through a 4k resistor (see Figure 3). The input to the buffer or the output of the reference is available at REF (Pin 3). The internal reference can be overdriven with an external reference if more accuracy is needed. The buffer output drives the internal DAC and is available at CAP (Pin 4). The CAP pin can be used to drive a steady DC load of less than 2mA. Driving an AC load is not recommended because it can cause the performance of the converter to degrade. Figure 3. Internal or External Reference Source – + 1605 • F03 INTERNAL CAPACITOR DAC BANDGAP REFERENCE VANA 4k 2.2 µF CAP (2.5V) 2.2 µF REF (2.5V) 4 3 For minimum code transition noise the REF pin and the CAP pin should each be decoupled with a capacitor to filter wideband noise from the reference and the buffer (2.2 µF tantalum). Offset and Gain Adjustments The LTC1605 offset and full-scale errors have been trimmed at the factory with the external resistors shown in Figure 4. This allows for external adjustment of offset and full scale in applications where absolute accuracy is important. See Figure 5 for the offset and gain trim circuit. First adjust the offset to zero by adjusting resistor R3. Apply an input voltage of –152.6mV (– 0.5LSB) and adjust R3 so the code is changing between 1111 1111 1111 1111 and 0000 0000 0000 0000. The gain error is trimmed by adjusting resistor R4. An input voltage of 9.999542V (+FS – 1.5LSB) is applied to VIN and R4 is adjusted until the output code is changing between 0111 1111 1111 1110 and 0111 1111 1111 1111. Figure 6 shows the bipolar transfer character- istic of the LTC1605. Figure 4. ±10V Input Without Trim + 5 4 3 2 1 2.2 µF 2.2 µF 33.2k 1% ±10V INPUT 200 Ω 1% VIN AGND1 REF CAP AGND2 LTC1605 1605 • F04 Figure 5. ±10V Input with Offset and Gain Trim INPUT VOLTAGE (V) 0V –1 LSB 1605 • F06 011...111 011...110 000...001 000...000 100...000 100...001 111...110 1 LSB BIPOLAR ZERO 111...111 FS/2 – 1LSB –FS/2 FS = 20V 1LSB = FS/65536 Figure 6. LTC1605 Bipolar Transfer Characteristics DC Performance One way of measuring the transition noise associated with a high resolution ADC is to use a technique where a DC + 5 4 3 2 1 2.2 µF 2.2 µF 33.2k 1% ±10V INPUT 200 Ω 1% VIN AGND1 REF CAP AGND2 LTC1605 1605 • F05 576k R4 50k R3 50k 5V |
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