Datasheet LTC1420 (Analog Devices) - 9

LTC1420
U U W U APPLICATIONS INFORMATION
VREF and VREF can be driven by an external voltage. With two additional resistors, VREF can be set to any voltage V TO REF ADC between 2.048V and 4.5V. 1µF + 1k An external reference or a DAC can be used to drive VREF over a 0V to 5V range (Figures 3a and 3b). The input R1 – 5k impedance of the VREF pin is 1k, so a buffer may be required for high accuracy. Driving VREF with a DAC is SENSE useful in applications where the peak input signal ampli- R2 tude may vary. The input span of the ADC can then be 5k LOGIC adjusted to match the peak input signal, maximizing the signal-to-noise ratio. 2.5V REFERENCE Both the VCM and VREF pins must be bypassed with 2.048V capacitors to ground. For best performance, 1µF or larger VCM ceramic capacitors are recommended. For the case of 1µF external circuitry driving VREF, a smaller capacitor can be 1420 F02 used at VREF so the input range can be changed quickly. In
Figure 2. Reference Circuit
this case, a 0.05µF or larger ceramic capacitor is accept- able. The VCM pin is a low output impedance 2.5V reference that 5V can be used by external circuitry. For single 5V supply VIN applications it is convenient to connect – AIN directly to the VOUT VREF VCM pin. LT1019A-2.5 1µF LTC1420
Driving the Analog Inputs
5V SENSE The differential inputs of the LTC1420 are easy to drive. VCM The inputs may be driven differentially or single-ended 1µF 1420 F03a (i. e., the – AIN input is held at a fixed value). The – AIN and + AIN inputs are simultaneously sampled and any com-
Figure 3a. Using the LT1019-2.5 As an
mon mode signal is reduced by the high common mode
External Reference; Input Range =
±
1.25V
rejection of the sample-and-hold circuit. Any common mode input value is acceptable as long as the input pins stay between VDD and VSS. During conversion, the analog LTC1420 inputs are high impedance. At the end of conversion, the + 2.048V inputs draw a small current spike while charging the VREF sample-and-hold. 1µF – 5k For superior dynamic performance in dual supply mode, SENSE the LTC1420 should be operated with the analog inputs 5k VCM centered at ground, and in single supply mode the inputs LTC1450 1µF 1420 F03b should be centered at 2.5V. If required, the analog inputs can be driven differentially via a transformer. Refer to Table 2 for a summary of the analog input and reference configurations and their relative advantages.
Figure 3b. Driving VREF with a DAC
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