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LTC1420C Datasheet(PDF) 9 Page - Linear Technology |
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LTC1420C Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page 9 LTC1420 VCM SENSE VREF 1420 F02 – + R1 5k LOGIC 2.5V REFERENCE 2.048V 1 µF 1 µF 1k TO ADC R2 5k VOUT VIN LT1019A-2.5 5V 1420 F03a 1 µF 1 µF VREF SENSE 5V VCM LTC1420 1420 F03b 1 µF 1 µF VREF LTC1420 SENSE VCM 2.048V – + 5k 5k LTC1450 Figure 2. Reference Circuit Figure 3a. Using the LT1019-2.5 As an External Reference; Input Range = ±1.25V Figure 3b. Driving VREF with a DAC APPLICATIONS INFORMATION VREF and VREF can be driven by an external voltage. With two additional resistors, VREF can be set to any voltage between 2.048V and 4.5V. An external reference or a DAC can be used to drive VREF over a 0V to 5V range (Figures 3a and 3b). The input impedance of the VREF pin is 1k, so a buffer may be required for high accuracy. Driving VREF with a DAC is useful in applications where the peak input signal ampli- tude may vary. The input span of the ADC can then be adjusted to match the peak input signal, maximizing the signal-to-noise ratio. Both the VCM and VREF pins must be bypassed with capacitors to ground. For best performance, 1 µF or larger ceramic capacitors are recommended. For the case of external circuitry driving VREF, a smaller capacitor can be used at VREF so the input range can be changed quickly. In this case, a 0.05 µF or larger ceramic capacitor is accept- able. The VCM pin is a low output impedance 2.5V reference that can be used by external circuitry. For single 5V supply applications it is convenient to connect – AIN directly to the VCM pin. Driving the Analog Inputs The differential inputs of the LTC1420 are easy to drive. The inputs may be driven differentially or single-ended (i. e., the – AIN input is held at a fixed value). The – AIN and +AIN inputs are simultaneously sampled and any com- mon mode signal is reduced by the high common mode 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 inputs are high impedance. At the end of conversion, the inputs draw a small current spike while charging the sample-and-hold. For superior dynamic performance in dual supply mode, the LTC1420 should be operated with the analog inputs centered at ground, and in single supply mode the inputs 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. |
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