Datasheet LTC1405 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | 12-Bit, 5Msps, Sampling ADC |
| Pages / Page | 20 / 10 — APPLICATIO S I FOR ATIO. Figure 2. Reference Circuit. Driving the Analog … |
| File Format / Size | PDF / 337 Kb |
| Document Language | English |
APPLICATIO S I FOR ATIO. Figure 2. Reference Circuit. Driving the Analog Inputs. Figure 3a. Using the LT1019-2.5 As an
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LTC1405
U U W U APPLICATIO S I FOR ATIO
An external reference or a DAC can be used to drive VREF over a 0V to 5V range (Figures 3a and 3b). The input V TO REF impedance of the V ADC REF pin is 2kΩ, so a buffer may be 1µF required for high accuracy. Driving VREF with a DAC is + 2k useful in applications where the peak input signal ampli- tude may vary. The input span of the ADC can then be R1 – 10k adjusted to match the peak input signal, maximizing the signal-to-noise ratio. SENSE Both the V R2 CM and VREF pins must be bypassed with 10k capacitors to ground. For best performance, 1µF or larger LOGIC ceramic capacitors are recommended. For the case of 2.5V external circuitry driving VREF, a smaller capacitor can be REFERENCE used at VREF so the input range can be changed quickly. In 2.048V this case, a 0.05µF or larger ceramic capacitor is accept- VCM able. 1µF The V 1405 F02 CM pin is a low output impedance 2.5V reference that can be used by external circuitry. For single 5V supply
Figure 2. Reference Circuit
applications it is convenient to connect A – IN directly to the VCM pin. 5V
Driving the Analog Inputs
VIN The differential inputs of the LTC1405 are easy to drive. VOUT VREF The inputs may be driven differentially or single-ended 1µF LT1019A-2.5 LTC1405 (i. e., the A – – IN input is held at a fixed value). The AIN and A + IN inputs are simultaneously sampled and any common 5V SENSE mode signal is reduced by the high common mode rejec- VCM tion of the sample-and-hold circuit. Any common mode 1µF input value is acceptable as long as the input pins stay 1405 F03a between VDD and VSS. During conversion the analog
Figure 3a. Using the LT1019-2.5 As an
inputs are high impedance. At the end of conversion the
External Reference; Input Range =
±
1.25V
inputs draw a small current spike while charging the sample-and-hold. For superior dynamic performance in dual supply mode, the LTC1405 should be operated with the analog inputs LTC1405 centered at ground, and in single supply mode the inputs + 2.048V VREF should be centered at 2.5V. If required, the analog inputs 1µF can be driven differentially via a transformer. Refer to – 5k Table 2 for a summary of the analog input and reference SENSE configurations and their relative advantages. 5k VCM LTC1450 1µF 1405 F03b
Figure 3b. Driving VREF with a DAC
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U U W U APPLICATIO S I FOR ATIO
An external reference or a DAC can be used to drive VREF over a 0V to 5V range (Figures 3a and 3b). The input V TO REF impedance of the V ADC REF pin is 2kΩ, so a buffer may be 1µF required for high accuracy. Driving VREF with a DAC is + 2k useful in applications where the peak input signal ampli- tude may vary. The input span of the ADC can then be R1 – 10k adjusted to match the peak input signal, maximizing the signal-to-noise ratio. SENSE Both the V R2 CM and VREF pins must be bypassed with 10k capacitors to ground. For best performance, 1µF or larger LOGIC ceramic capacitors are recommended. For the case of 2.5V external circuitry driving VREF, a smaller capacitor can be REFERENCE used at VREF so the input range can be changed quickly. In 2.048V this case, a 0.05µF or larger ceramic capacitor is accept- VCM able. 1µF The V 1405 F02 CM pin is a low output impedance 2.5V reference that can be used by external circuitry. For single 5V supply
Figure 2. Reference Circuit
applications it is convenient to connect A – IN directly to the VCM pin. 5V
Driving the Analog Inputs
VIN The differential inputs of the LTC1405 are easy to drive. VOUT VREF The inputs may be driven differentially or single-ended 1µF LT1019A-2.5 LTC1405 (i. e., the A – – IN input is held at a fixed value). The AIN and A + IN inputs are simultaneously sampled and any common 5V SENSE mode signal is reduced by the high common mode rejec- VCM tion of the sample-and-hold circuit. Any common mode 1µF input value is acceptable as long as the input pins stay 1405 F03a between VDD and VSS. During conversion the analog
Figure 3a. Using the LT1019-2.5 As an
inputs are high impedance. At the end of conversion the
External Reference; Input Range =
±
1.25V
inputs draw a small current spike while charging the sample-and-hold. For superior dynamic performance in dual supply mode, the LTC1405 should be operated with the analog inputs LTC1405 centered at ground, and in single supply mode the inputs + 2.048V VREF should be centered at 2.5V. If required, the analog inputs 1µF can be driven differentially via a transformer. Refer to – 5k Table 2 for a summary of the analog input and reference SENSE configurations and their relative advantages. 5k VCM LTC1450 1µF 1405 F03b
Figure 3b. Driving VREF with a DAC
1405fa 10
