Driving the Analog Input of the HI5812 Family of 12-Bit Analog to Digital Converters TM Technical Brief December 2001 TB335.1 Author: Juan C. Garcia By selecting the values of R2 and R3, the user can properly DC offset the bipolar signal to bring it within the operating range of the HI5812 converter. Since the full scale range of the HI5812 family is smaller than the 574 series, which is 10V in unipolar mode and 20V in bipolar mode, the input drive circuit must attenuate the incoming signal to the converter, allowing the user to maintain the existing circuitry. Resistors R A and RB can be selected to accomplish this task with relative ease. As seen in the equation below, RA and RB can be set to accommodate the 5V full scale range of the HI5812 converter by choosing a 3:1 ratio (when going from 10V FSR to 5V FSR) or a 7:1 ratio (when going from 20V FSR to 5V FSR). These ratios may need to be modified depending on the gain configuration of the overall circuit. The HI5812 Analog to Digital converter offers system designers a high degree of flexibility combined with low power and low cost. When compared to the industry standard X74 (574, 674 and 774) series of A/D converters, the HI5812 represents an attractive alternative when digitizing low speed analog signals. With its on board track and hold circuitry (which is not present on the X74 family) and throughput rates of 50 KSPS, as well as single supply operation, the HI5812 represents a viable, low cost alternative solution for 12-bit ADC needs. The HI5812 operates on a single 5V power supply. When handling unipolar signals (those occurring above ground exclusively), this offers designers a near drop-in replacement over the 574 series. However when dealing with bipolar signals (those occurring both above and below the ground reference), the analog signal must be level shifted to allow the converter to properly digitize the incoming signal. A simple circuit such as the one shown in Figure 1 is ideal for accomplishing this task. The transfer function for this circuit is listed below. V IN1 = RB ---------------------- R A + R B × VI N Since the input impedance as seen by the outside world is the combination of RA and RB (due to the high input impedance of the amplifier), the only limitation when choosing these resistors is making sure that maximum power dissipation specifications are met. Table 1 lists recommended values for these resistors per input voltage range. R2 R 2 V OU T = 1 + ------------------- VIN1 – ------- VR R R R 3 1 3 +V VIN HI5812 RA VIN1 RB VOUT + R3 VIN R2 100Ω R1 VR FIGURE 1. HI5812 INPUT DRIVE CIRCUIT 1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 321-724-7143 | Intersil (and design) is a trademark of Intersil Americas Inc. Copyright © Intersil Americas Inc. 2001. All Rights Reserved Technical Brief 335 TABLE 1. RECOMMENDED RESISTOR VALUES UNIPOLAR INPUT BIPOLAR INPUT VIN 0V to +10V -10V to +10V VIN1 0V to +2.5V -1.25V to +1.25V VOUT 0V to 5V 0V to 5V RA 3kΩ 7kΩ RB 1kΩ 1kΩ VR N/A -5V R1 1kΩ 2kΩ R2 1kΩ 1kΩ R3 N/A 2kΩ When adapting this circuit for use in driving the input of the HI5812, several considerations should be made to assure proper performance is achieved. The input of the converter is highly capacitive (due to the switch capacitor nature of the design) and requires a minimum of 20mA of output drive capability on the amplifier. The amplifier must also be able to settle to 0.05% within a 2.3µs window when driving a relatively small load (~75Ω created by the input of the converter and the 100Ω load resistor on the output of the amplifier). The final consideration is the offset voltage of the amplifier. Though this in and of itself is not of major concern, it should be accounted for when adjusting the overall offset of the drive circuit. Since the HI5812 has an LSB size of 1mV, excessive offset voltage on the part of the amplifier can cut into the dynamic range of the converter. This condition could cause the converter to clip the upper and/or lower portion of the input signal. Refer to any HI5812 data sheet for the recommended methods of adjusting out offset errors. Intersil offers a variety of amplifiers capable of meeting the needs of the HI5812 such as: HA-2841, HA-5221, HA-5020, HFA1100, HFA1155. All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. 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