Datasheet LTC2439-1 (Analog Devices) - 9

LTC2439-1
APPLICATIONS INFORMATION Conversion Clock
specifications of the device if the power supply voltage is A major advantage the delta-sigma converter offers over restored within the operating range (2.7V to 5.5V) before conventional type converters is an on-chip digital filter the end of the POR time interval. (commonly implemented as a Sinc or Comb filter). For
Reference Voltage Range
high resolution, low frequency applications, this filter is typically designed to reject line frequencies of 50Hz and The LTC2439-1 accepts a truly differential external refer- 60Hz plus their harmonics. The filter rejection performance ence voltage. The absolute/common mode voltage specifi- is directly related to the accuracy of the converter system cation for the REF+ and REF– pins covers the entire range clock. The LTC2439-1 incorporates a highly accurate from GND to VCC. For correct converter operation, the on-chip oscillator. This eliminates the need for external REF+ pin must always be more positive than the REF– pin. frequency setting components such as crystals or oscil- The LTC2439-1 can accept a differential reference voltage lators. Clocked by the on-chip oscillator, the LTC2436-1 from 0.1V to V achieves a minimum of 87dB rejection over the range CC. The converter output noise is deter- mined by the thermal noise of the front-end circuits, and 49Hz to 61.2Hz. as such, its value in microvolts is nearly constant with reference voltage. A decrease in reference voltage will
Ease of Use
significantly improve the converter’s effective resolution, The LTC2439-1 data output has no latency, filter settling since the thermal noise (1µV) is well below the quan- delay or redundant data associated with the conversion tization level of the device (75.6µV for a 5V reference). cycle. There is a one-to-one correspondence between the At the minimum reference (100mV) the thermal noise conversion and the output data. Therefore, multiplexing remains constant at 1µV RMS (or 6µVP-P), while the multiple analog voltages is easy. quantization is reduced to 1.5µV per LSB. As a result, The LTC2439-1 performs offset and full-scale calibrations lowering the reference improves the effective resolution in every conversion cycle. This calibration is transparent to for low level input voltages. the user and has no effect on the cyclic operation described
Input Voltage Range
above. The advantage of continuous calibration is extreme stability of offset and full-scale readings with respect to The two selected pins are labeled IN+ and IN– (see Table time, supply voltage change and temperature drift. 1). Once selected (either differential or single-ended multiplexing mode), the analog input is differential with
Power-Up Sequence
a common mode range for the IN+ and IN– input pins The LTC2439-1 automatically enters an internal reset extending from GND – 0.3V to VCC + 0.3V. Outside these state when the power supply voltage V limits, the ESD protection devices begin to turn on and CC drops below approximately 2V. This feature guarantees the integrity the errors due to input leakage current increase rapidly. of the conversion result and of the serial interface mode Within these limits, the LTC2439-1 converts the bipolar selection. (See the 3-wire I/O sections in the Serial Interface differential input signal, VIN = IN+ – IN–, from –FS = –0.5 Timing Modes section.) • VREF to +FS = 0.5 • VREF where VREF = REF+ – REF–. Outside this range the converter indicates the overrange When the VCC voltage rises above this critical threshold, or the underrange condition using distinct output codes. the converter creates an internal power-on-reset (POR) signal with a typical duration of 1ms. The POR signal Input signals applied to IN+ and IN– pins may extend clears all internal registers. Following the POR signal, 300mV below ground or above VCC. In order to limit any the LTC2439-1 starts a normal conversion cycle and fault current, resistors of up to 5k may be added in series follows the succession of states described above. The first with the IN+ or IN– pins without affecting the performance conversion result following POR is accurate within the of the device. In the physical layout, it is important to main- 24391fb For more information www.linear.com/LTC2439-1 9 Document Outline Features Applications Typical Application Absolute Maximum Ratings Order Information Electrical Characteristics Converter Characteristics Analog Input and Reference Digital Inputs and Digital Outputs POWER REQUIREMENTS Timing Characteristics Pin Functions FUNCTIONAL Block Diagram Test CircuitS Applications Information Package Description Revision History Typical Application Related Parts