Datasheet LTC1410 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | 12-Bit, 1.25Msps, Sampling A/D Converter with Shutdown |
| Pages / Page | 16 / 10 — APPLICATIONS INFORMATION. Figure 7. RC Input Filter. Internal Reference. … |
| File Format / Size | PDF / 162 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Figure 7. RC Input Filter. Internal Reference. Figure 6. Acquisition Time vs Source Resistance
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LTC1410
U U W U APPLICATIONS INFORMATION
capacitors at the end of conversion. During conversion sample-and-hold circuit is 20MHz. Any noise that is present the analog inputs draw only a small leakage current. If the at the analog inputs will be summed over this entire source impedance of the driving circuit is low then the bandwidth. Noisy input circuitry should be filtered prior to LTC1410 inputs can be driven directly. As source imped- the analog inputs to minimize noise. A simple 1-pole RC ance increases so will acquisition time (see Figure 6). For filter is usually sufficient. For example, Figure 7 shows a minimum acquisition time with high source impedance, 1000pF capacitor from + AIN to ground and a 100Ω source a buffer amplifier should be used. The only requirement resistor will limit the input bandwidth to 1.6MHz. Simple is that the amplifier driving the analog input(s) must RC filters work well for AC applications, but they will limit settle after the small current spike before the next conver- the transient response. For full speed operation, amplifiers sion starts (settling time must be 100ns for full through- with fast settling and low noise should be chosen. put rate). 100Ω 10 ANALOG 1 +A INPUT IN 1000pF 2 –AIN µs) 3 1 VREF 4 REFCOMP 10µF 0.1µF 5 AGND 0.1 ACQUISITION TIME ( LTC1410 1410 F07 0.0110 100 1k 10k 100k
Figure 7. RC Input Filter
SOURCE RESISTANCE (Ω) 1410 F06
Internal Reference Figure 6. Acquisition Time vs Source Resistance
The LTC1410 has an on-chip, temperature compensated, curvature corrected, bandgap reference which is factory Choosing an input amplifier is easy if a few requirements trimmed to 2.500V. It is connected internally to a reference are taken into consideration. First, choose an amplifier amplifier and is available at VREF (Pin 3). See Figure 8a. A that has a low output impedance (< 100Ω) at the closed- 2k resistor is in series with the output so that it can be loop bandwidth frequency. For example, if an amplifier is used in a gain of +1 and has a closed-loop bandwidth of 50MHz, then the output impedance at 50MHz must be 1 ANALOG +AIN LTC1410 INPUT less than 100Ω. The second requirement is that the 2 closed-loop bandwidth must be greater than 20MHz to –AIN R1 ensure adequate small-signal settling for full throughput 3 V 2k REF BANDGAP 2.500V REFERENCE rate. If slower op amps are used, more settling time can 4.06V REFCOMP + 4 be provided by increasing the time between conversions. R2 – Suitable devices capable of driving the ADC’s inputs 10µF 0.1µF 40k include the LT®1360, LT1220, LT1223, LT1224 and 5 AGND LT1227 op amps. R3 64k The noise and the distortion of the input amplifier must 1410 F08a also be considered since they will add to the LTC1410 noise and distortion. The small-signal bandwidth of the
Figure 8a. LTC1410 Reference Circuit
10
U U W U APPLICATIONS INFORMATION
capacitors at the end of conversion. During conversion sample-and-hold circuit is 20MHz. Any noise that is present the analog inputs draw only a small leakage current. If the at the analog inputs will be summed over this entire source impedance of the driving circuit is low then the bandwidth. Noisy input circuitry should be filtered prior to LTC1410 inputs can be driven directly. As source imped- the analog inputs to minimize noise. A simple 1-pole RC ance increases so will acquisition time (see Figure 6). For filter is usually sufficient. For example, Figure 7 shows a minimum acquisition time with high source impedance, 1000pF capacitor from + AIN to ground and a 100Ω source a buffer amplifier should be used. The only requirement resistor will limit the input bandwidth to 1.6MHz. Simple is that the amplifier driving the analog input(s) must RC filters work well for AC applications, but they will limit settle after the small current spike before the next conver- the transient response. For full speed operation, amplifiers sion starts (settling time must be 100ns for full through- with fast settling and low noise should be chosen. put rate). 100Ω 10 ANALOG 1 +A INPUT IN 1000pF 2 –AIN µs) 3 1 VREF 4 REFCOMP 10µF 0.1µF 5 AGND 0.1 ACQUISITION TIME ( LTC1410 1410 F07 0.0110 100 1k 10k 100k
Figure 7. RC Input Filter
SOURCE RESISTANCE (Ω) 1410 F06
Internal Reference Figure 6. Acquisition Time vs Source Resistance
The LTC1410 has an on-chip, temperature compensated, curvature corrected, bandgap reference which is factory Choosing an input amplifier is easy if a few requirements trimmed to 2.500V. It is connected internally to a reference are taken into consideration. First, choose an amplifier amplifier and is available at VREF (Pin 3). See Figure 8a. A that has a low output impedance (< 100Ω) at the closed- 2k resistor is in series with the output so that it can be loop bandwidth frequency. For example, if an amplifier is used in a gain of +1 and has a closed-loop bandwidth of 50MHz, then the output impedance at 50MHz must be 1 ANALOG +AIN LTC1410 INPUT less than 100Ω. The second requirement is that the 2 closed-loop bandwidth must be greater than 20MHz to –AIN R1 ensure adequate small-signal settling for full throughput 3 V 2k REF BANDGAP 2.500V REFERENCE rate. If slower op amps are used, more settling time can 4.06V REFCOMP + 4 be provided by increasing the time between conversions. R2 – Suitable devices capable of driving the ADC’s inputs 10µF 0.1µF 40k include the LT®1360, LT1220, LT1223, LT1224 and 5 AGND LT1227 op amps. R3 64k The noise and the distortion of the input amplifier must 1410 F08a also be considered since they will add to the LTC1410 noise and distortion. The small-signal bandwidth of the
Figure 8a. LTC1410 Reference Circuit
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