Datasheet LTC1857, LTC1858, LTC1859 (Analog Devices) - 10

LTC1857/LTC1858/LTC1859
OPERATION OVERVIEW
Before starting a conversion, an 8-bit data word is clocked The LTC1857/LTC1858/LTC1859 are innovative, multichan- into the SDI input on the first eight rising SCK edges to nel ADCs that provide software-selectable input ranges for select the MUX address, input range and power down each of their eight input channels. Using on-chip resistors mode. The ADC enters acquisition mode on the falling and switches, it provides an attenuation and offset that can edge of the sixth clock in the 8-bit data word and ends be programmed for each channel on the fly. The precisely on the rising edge of the CONVST signal which also starts trimmed attenuators ensure accurate input ranges. Because a conversion (see Figure 7). A minimum time of 4µs will they precede the multiplexer, errors due to multiplexer provide enough time for the sample-and-hold capacitors on-resistance are eliminated. to acquire the analog signal. Once a conversion cycle has begun, it cannot be restarted. The input word that selects the input channel also selects the desired input range for that channel. The available ranges During the conversion, the internal differential 12-/14- are 0V to 5V, 0V to 10V (unipolar), ±5V and ±10V (bipolar). /16-bit capacitive DAC output is sequenced by the SAR from They are achieved with the ADC running on a single 5V the most significant bit (MSB) to the least significant bit supply. In addition to the range selection, single-ended (LSB). The input is successively compared with the binary or differential inputs may be selected for each channel or weighted charges supplied by the differential capacitive pair of channels. Finally, overrange protection is provided DAC. Bit decisions are made by a high speed comparator. for unselected channels. An overrange condition on an At the end of a conversion, the DAC output balances the unused channel will not affect the conversion result on analog input (ADC+ – ADC–). The SAR contents (a 16-bit the selected channel. data word) which represents the difference of ADC+ and ADC– are loaded into the 12-/14-/16-bit shift register.
CONVERSION DETAILS DRIVING THE ANALOG INPUTS
The LTC1857/LTC1858/LTC1859 use a successive ap- proximation algorithm and an internal sample-and-hold The nominal input ranges for the LTC1857/LTC1858/ circuit to convert an analog signal to a 12-/14-/16-bit serial LTC1859 are 0V to 5V, 0V to 10V, ±5V and ±10V and the output respectively. The ADCs are complete with a precision MUX inputs are overvoltage protected to ±25V. The input reference and an internal clock. The control logic provides impedance is typically 42kΩ in unipolar mode and 31kΩ easy interface to microprocessors and DSPs. (Please refer in bipolar mode, therefore, it should be driven with a low to the Digital Interface section for the data format.) impedance source. Wideband noise coupling into the input can be minimized by placing a 3000pF capacitor at the input The analog signals applied at the MUX input channels are as shown in Figure 2. An NPO-type capacitor gives the rescaled by the resistor divider network formed by R1, R2 lowest distortion. Place the capacitor as close to the device and R3 as shown below. The rescaled signals appear on input pin as possible. If an amplifier is to be used to drive the MUXOUT (Pins 10, 11) which are also connected to the input, care should be taken to select an amplifier with the ADC inputs (Pins 12, 13) under normal operation. adequate accuracy, linearity and noise for the application. The following list is a summary of the op amps that are REFCOMP suitable for driving the LTC1857/LTC1858/LTC1859. More BIPOLAR detailed information is available in the Linear Technology data books and online at www.linear.com. R1 R3 CH SEL 10k MUX 25k LT®1007: Low noise precision amplifier. 2.7mA supply INPUT MUXOUT R2 current ±5V to ±15V supplies. Gain bandwidth product 17k 8MHz. DC applications. 1859 AI03 185789fb 10 For more information www.linear.com/LTC1857 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Converter and Multplexer Characteristics Analog Input Dynamic Accuracy Internal Reference Characteristics Digital Inputs and Digital Outputs Power Requirements Timing Characteristics Typical Performance Characteristics Pin Functions Functional Block Diagram Test Circuits Timing Diagrams Operation Applications Information Package Description Revision History Typical Application Related Parts