Datasheet MAX912, MAX913 (Analog Devices) - 7

Single/Dual, Ultra-Fast, Low-Power Precision TTL Comparators MAX912/MAX913 Detailed Description
+25°C. Input common-mode range extends from 200mV
below
the negative supply rail to 1.5V below The MAX912 (dual) and MAX913 (single) high-speed the positive power supply. The total common-mode comparators have a unique design that prevents oscil- range is 8.7V when operating from ±5VDC supplies. lation when the comparator is in its linear region. No minimum input slew rate is required. The MAX912/MAX913’s amplifier has no built-in hys- teresis. For highest accuracy, do not add hysteresis. Many high-speed comparators oscillate in the linear Figure 2 shows how hysteresis degrades resolution. region, as shown in the Typical Operating Characteris- tics’ industry-standard 686 response graph. One way
Resolution
to overcome this oscillation is to sample the output after A comparator’s ability to resolve small signal differ- it has passed through the unstable region. Another ences—its resolution—is affected by various factors. practical solution is to add hysteresis. Either solution As with most amplifiers, the most significant factors are results in a loss of resolution and bandwidth. the input offset voltage (VOS) and the common-mode Because the MAX912/MAX913 do not need hysteresis, and power-supply rejection ratios (CMRR, PSRR). If they offer high resolution to all signals—including low- source impedance is high, input offset current can be frequency signals. significant. If source impedance is unbalanced, the input bias current can introduce another error. The MAX912/MAX913 provide a TTL-compatible latch function that holds the comparator output state (Figure 1). For high-speed comparators, an additional factor in As long as Latch Enable (LE) is high or floating, the input resolution is the comparator’s stability in its linear signal has no effect on the output state. With LE low, the region. Many high-speed comparators are useless in outputs are controlled by the input differential voltage their linear region because they oscillate. This makes and the latch is transparent. the differential input voltage region around 0V unus- able, as does a high VOS. Hysteresis does not cure the
Input Amplifier
problem, but acts to keep the input away from its linear A comparator can be thought of as having two sec- range (Figure 2). tions; an input amplifier and a logic interface. The The MAX912/MAX913 do not oscillate in the linear MAX912/MAX913’s input amplifier is fully differential region, which greatly enhances the comparator’s reso- with input offset voltage trimmed to below 2.0mV at lution. tSU VIN (DIFFERENTIAL) tH LATCH ENABLE (LE) t t PD+ LPD Q ∆tPD Q tPD- Figure 1. Timing Diagram
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