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MAX951 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX951 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page In the example, RA is again 24k Ω. 6) Select the upper trip point VS(H). Our example is set at 4.75V. 7) Calculate RB. RB is 8.19k Ω, or approximately 8.2kΩ. Input Noise Considerations Because low power requirements often demand high- impedance circuits, effects from radiated noise are more significant. Thus, traces between the op-amp or com- parator inputs and any resistor networks attached should be kept as short as possible. Crosstalk Reference Internal crosstalk to the reference from the comparator is package dependent. Typical values (VDD = 5V) are 45mV for the plastic DIP package and 32mV for the SO package. Applications using the reference for the op amp or external circuitry can eliminate this crosstalk by using a simple RC lowpass filter, as shown in Figure 5. Op Amp Internal crosstalk to the op amp from the comparator is package dependent, but not input referred. Typical val- ues (VDD = 5V) are 4mV for the plastic DIP package and 280µV for the SO package. Op-Amp Stability and Board Layout Considerations Unlike other industry-standard micropower CMOS op amps, the op amps in the MAX951–MAX954 maintain stability in their minimum gain configuration while driving heavy capacitive loads, as demonstrated in the MAX951/MAX953 Op-Amp Percent Overshoot vs. Capacitive Load graph in the Typical Operating Characteristics. Although this family is primarily designed for low-fre- quency applications, good layout is extremely impor- tant. Low-power, high-impedance circuits may increase the effects of board leakage and stray capacitance. For example, the combination of a 10M Ω resistance (from leakage between traces on a contaminated, poorly designed PC board) and a 1pF stray capacitance pro- vides a pole at approximately 16kHz, which is near the amplifier’s bandwidth. Board routing and layout should minimize leakage and stray capacitance. In some cases, stray capacitance may be unavoidable and it may be necessary to add a 2pF to 10pF capacitor across the feedback resistor to compensate; select the smallest capacitor value that ensures stability. Input Overdrive With 100mV overdrive, comparator propagation delay is typically 6µs. The Typical Operating Characteristics show propagation delay for various overdrive levels. Supply current can increase when the op amp in the MAX951–MAX954 is overdriven to the negative supply rail. For example, when connecting the op amp as a compara- tor and applying a -100mV input overdrive, supply current rises by around 15µA and 32µA for supply voltages of 2.8V and 7V, respectively. R V V R R R V V V R R B REF IHYST A S H REF IHSYT A = + ( ) ( )( ) ( ) − + ( ) + ( ) ( ) 2 2 2 R R V V for V V A SHYST DD SHYST IHYST ≈ >> 2 , Ultra-Low-Power, Single-Supply Op Amp + Comparator + Reference 10 ______________________________________________________________________________________ R2 R1 2pF to 10pF AMPOUT AMPIN+ REF 0.1 µF 0.1 µF L1 330mH C1A 390pF 1 (2 π fC) 2 C1B 330pF C1C 20-60pF 10M 5.1M 1.0M 100k 1.2V 20k VCC = 5V COMP ANTENNA AMP LAYOUT-SENSITIVE AREA, METAL RFI SHIELDING ADVISED MAX952 L1 x C1 = Figure 3. Compensation for Feedback-Node Capacitance Figure 4. Low-Frequency Radio Receiver Application |
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