Datasheet LTC1287 (Analog Devices) - 8

LTC1287
O U U W U APPLICATI S I FOR ATIO
specified at 3.3V. The output DOUT is also compatible with
ANALOG CONSIDERATIONS
the above standards. The following summarizes such
Grounding
levels. The LTC1287 should be used with an analog ground plane VOH (no load) VCC – 0.1V and single point grounding techniques. Do not use wire VOL (no load) 0.1V wrapping techniques to breadboard and evaluate the device. VOH 0.9 × VCC To achieve the optimum performance use a PC board. The VOL 0.1 × VCC ground pin (Pin 4) should be tied directly to the ground VIH 0.7 × VCC plane with minimum lead length (a low profile socket is VIL 0.2 × VCC fine). Pin 7 (VCC) should be bypassed to the ground plane The LTC1287 can be driven with 5V logic even when V with a 22µF (minimum value) tantalum with leads as short CC is at 3.3V. This is due to a unique input protection device as possible and as close as possible to the pin. A 0.1µF that is found on the LTC1287. ceramic disk also should be placed in parallel with the 22µF and again with leads as short as possible and as close
Microprocessor Interfaces
to VCC as possible. Figure 3 shows an example of an ideal LTC1287 ground plane design for a two-sided board. Of The LTC1287 can interface directly (without external hard- course this much ground plane will not always be possible, ware) to most popular microprocessor (MPU) synchro- but users should strive to get as close to this ideal as nous serial formats. If an MPU without a serial interface is possible. used, then three of the MPU’s parallel port lines can be programmed to form the serial link to the LTC1287. Many
Bypassing
of the popular MPUs can operate with 3V supplies. For example the MC68HC11 is an MPU with a serial format For good performance, VCC must be free of noise and (SPI). Likewise parallel MPUs that have the 8051 type ripple. Any changes in the VCC voltage with respect to architecture are also capable of operating at this voltage ground during a conversion cycle can induce errors or range. The code for these processors remains the same noise in the output code. VCC noise and ripple can be kept and can be found in the LTC1292 data sheet. below 0.5mV by bypassing the VCC pin directly to the analog plane with a minimum of 22µF tantalum capacitor
Sharing the Serial Interface
and with leads as short as possible. The lead from the device to the V The LTC1287 can share the same two-wire serial interface CC supply also should be kept to a minimum and the V with other peripheral components or other LTC1287s CC supply should have a low output impedance (Figure 2). In this case, the CS signals decide which LTC1287 is being addressed by the MPU. VCC 0.1µF 22µF TANTALUM 2 1 0 OUTPUT PORT 2-WIRE SERIAL 1 8 2 SERIAL DATA INTERFACE TO OTHER 2 2 2 7 2 PERIPHERALS OR LTC1287s LTC1287 3 6 CS CS CS MPU LTC1287 LTC1287 LTC1287 4 5 LTC1287 F03 2 CHANNELS 2 CHANNELS 2 CHANNELS LTC1287 F02
Figure 2. Several LTC1287s Sharing One 2-Wire Serial Interface Figure 3. Example Ground Plane for the LTC1287
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