Datasheet LTC1040 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | Dual Micropower Comparator |
| Pages / Page | 12 / 7 — APPLICATIO S I FOR ATIO. Offset Voltage Error. Pulsed Power (V. P-P) … |
| File Format / Size | PDF / 227 Kb |
| Document Language | English |
APPLICATIO S I FOR ATIO. Offset Voltage Error. Pulsed Power (V. P-P) Output. Figure 6. Ratiometric Network Driven by VP-P
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LTC1040
U U W U APPLICATIO S I FOR ATIO Offset Voltage Error
The VP-P output voltage is not precise (see VP-P Output The errors due to offset, common mode, power supply Voltage versus Load Current curve). There are two ways variation, gain and temperature are all included in the VP-P can be used to power external networks without offset voltage specification. This makes it easy to compute excessive errors: (1) ratiometric networks and (2) fast the error when using the LTC1040. settling references. Example: error computation for Figure 4. In a ratiometric network, the inputs are all proportional to Assume: 2.8V ≤ V VP-P (see Figure 6). Consequently, for small changes, the S ≤ 6V. absolute value of VP-P does not affect accuracy. Then total worst-case error is: It is critical that the inputs to the LTC1040 completely 1A settle within 4µs of the start of the comparison cycle and I = L (ERROR) = ± (100mV • 0.001 + 0.5mV) • ±6mA 100mV that they do not change during the 80µs ON time. When driving resistive networks with VP-P, capacitive loading on ↑ ↑ Tracking Error VOS 6mA VP-P OUTPUT IL (ERROR)% = • 100 = ± 0.6%. 1A – VIN + Note: If source resistance exceeds 10k, bypass + capacitors should be used and the associated errors must – 1/2 OUTPUT VTRIP + LTC1040 be included. –
Pulsed Power (V
LTC1040 • AI06
P-P) Output Figure 6. Ratiometric Network Driven by VP-P
It is often desirable to use comparators with resistive networks such as bridges. Because of the extremely low the network should be minimized to meet the 4µs settling power consumption of the LTC1040, the power consumed time requirement. It is not recommended that VP-P be used by these resistive networks can far exceed that of the to drive networks with source impedances, as seen by the device itself. inputs, of greater than 10kΩ. At low sample rates the LTC1040 spends most of its time In applications where an absolute reference is required, off. To take advantage of this, a pulsed power (VP-P) output the VP-P output can be used to drive a fast settling is provided. VP-P is switched to V+ when the comparator reference. The LT1009 2.5V reference, ideal in this is on and to a high impedance (open circuit) when the application, settles in approximately 2µs (see Figure 7). comparator is off. The ON time is nominally 80µs. The current through R1 must be large enough to supply Figure 5 shows the VP-P output circuit. the LT1009 minimum bias current (≈1mA) and the load current, IL. V+ 18 VP-P OUTPUT R1 + Q1 P1 VIN – 1/2 R2 + LTC1040 80µs I – L COMPARATOR ON TIME LT1009 R3 9 17 GND VP-P LTC1040 • AI07 LTC1040 • AI05
Figure 5. V Figure 7. Driving Reference with V P-P Output Switch P-P Output
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U U W U APPLICATIO S I FOR ATIO Offset Voltage Error
The VP-P output voltage is not precise (see VP-P Output The errors due to offset, common mode, power supply Voltage versus Load Current curve). There are two ways variation, gain and temperature are all included in the VP-P can be used to power external networks without offset voltage specification. This makes it easy to compute excessive errors: (1) ratiometric networks and (2) fast the error when using the LTC1040. settling references. Example: error computation for Figure 4. In a ratiometric network, the inputs are all proportional to Assume: 2.8V ≤ V VP-P (see Figure 6). Consequently, for small changes, the S ≤ 6V. absolute value of VP-P does not affect accuracy. Then total worst-case error is: It is critical that the inputs to the LTC1040 completely 1A settle within 4µs of the start of the comparison cycle and I = L (ERROR) = ± (100mV • 0.001 + 0.5mV) • ±6mA 100mV that they do not change during the 80µs ON time. When driving resistive networks with VP-P, capacitive loading on ↑ ↑ Tracking Error VOS 6mA VP-P OUTPUT IL (ERROR)% = • 100 = ± 0.6%. 1A – VIN + Note: If source resistance exceeds 10k, bypass + capacitors should be used and the associated errors must – 1/2 OUTPUT VTRIP + LTC1040 be included. –
Pulsed Power (V
LTC1040 • AI06
P-P) Output Figure 6. Ratiometric Network Driven by VP-P
It is often desirable to use comparators with resistive networks such as bridges. Because of the extremely low the network should be minimized to meet the 4µs settling power consumption of the LTC1040, the power consumed time requirement. It is not recommended that VP-P be used by these resistive networks can far exceed that of the to drive networks with source impedances, as seen by the device itself. inputs, of greater than 10kΩ. At low sample rates the LTC1040 spends most of its time In applications where an absolute reference is required, off. To take advantage of this, a pulsed power (VP-P) output the VP-P output can be used to drive a fast settling is provided. VP-P is switched to V+ when the comparator reference. The LT1009 2.5V reference, ideal in this is on and to a high impedance (open circuit) when the application, settles in approximately 2µs (see Figure 7). comparator is off. The ON time is nominally 80µs. The current through R1 must be large enough to supply Figure 5 shows the VP-P output circuit. the LT1009 minimum bias current (≈1mA) and the load current, IL. V+ 18 VP-P OUTPUT R1 + Q1 P1 VIN – 1/2 R2 + LTC1040 80µs I – L COMPARATOR ON TIME LT1009 R3 9 17 GND VP-P LTC1040 • AI07 LTC1040 • AI05
Figure 5. V Figure 7. Driving Reference with V P-P Output Switch P-P Output
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