The MAX913 (single) and MAX912 (dual) high-speed comparators have a unique design that prevents oscillation when the comparator is in its linear region. No minimum input slew rate is required. Many high-speed comparators oscillate in the linear region, as shown in the Typical Operating Characteristics industry-standard 686 response graph. One way to overcome this oscillation is to sample the output after it has passed through the unstable region. Another practical solution is to add hysteresis. Either solution results in a loss of resolution and bandwidth. Because the MAX912/MAX913 do not need hysteresis, they offer high resolution to all signals—including low frequency signals.
The MAX912/MAX913 provide a TTL-compatible latch function that holds the comparator output state. As long as Latch Enable (LE) is high or floating, the input signal has no effect on the output state. With LE low, the outputs are controlled by the input differential voltage and the latch is transparent.
A comparator can be thought of as having two sections: an input amplifier and a logic interface. The MAX912/MAX913’s input amplifier is fully differential, with input offset voltage trimmed to below 2.0mV at +25°C. Input common-mode range extends from 200mV below the negative supply rail to 1.5V below the positive power supply. The total common-mode range is 8.7V when operating from ±5VDC supplies. The MAX912/MAX913’s amplifier has no built-in hysteresis. For highest accuracy, do not add hysteresis.
A comparator’s ability to resolve small signal differences-its resolution-is affected by various factors. As with most amplifiers, the most significant factors are the input offset voltage (VOS) and the common-mode and power-supply rejection ratios (CMRR, PSRR). If source impedance is high, input offset current can be significant. If source impedance is unbalanced, the input bias current can introduce another error. For high-speed comparators, an additional factor in resolution is the comparator’s stability in its linear region. Many high-speed comparators are useless in their linear region because they oscillate. This makes the differential input voltage region around 0V unusable, as does a high VOS. Hysteresis does not cure the problem, but acts to keep the input away from its linear range. The MAX912/MAX913 do not oscillate in the linear region, which greatly enhances the comparator’s resolution.
- Ultra Fast (10ns)
- Single +5V or Dual ±5V Supply Operation
- Input Range Extends Below Negative Supply
- Low Power: 6mA (+5V) Per Comparator
- No Minimum Input Signal Slew-Rate Requirement
- No Power-Supply Current Spiking
- Stable in the Linear Region
- Inputs Can Exceed Either Supply
- Low Offset Voltage: 0.8mV
- Zero-Crossing Detectors
- Ethernet Line Receivers
- Switching Regulators
- High-Speed Sampling Circuits
- High-Speed Triggers
- Extended Range V/F Converters
- Fast Pulse Width/Height Discriminators
Power Supplies and Bypassing
The MAX912/MAX913 are tested with ±5V power supplies that provide an input common-mode range (VCM) of 8.7V (-5.2V to +3.5V). Operation from a single +5V supply provides a common-mode input range of 3.7V (-0.2V to +3.5V). Connect V- to GND for single-supply operation. The MAX912/MAX913 will operate from a minimum single-supply voltage of +4.5V. The V+ supply provides power to both the analog input stage and digital output circuits, whereas the V- supply only powers the analog section. Bypass V+ and V- to ground with 0.1μF to 1.0μF ceramic capacitors in parallel with 10μF or greater tantalum capacitors. Connect the ceramic capacitors very close to the MAX912/MAX913’s supply pins, keeping leads short to minimize lead inductance. For particularly noisy applications, use ferrite beads on the power-supply lines.
Input Slew Rate
The MAX912/MAX913 design eliminates the input slew rate requirement imposed on many standard comparators. As long as LE is high after the maximum propagation delay and the input is greater than the comparator total DC error, the output will be valid without oscillations.
Maximum Clock (LE) and Signal Rate
The maximum clock and signal rate is 70MHz, based on the comparator’s rise and fall time with a 5mV overdrive at +25°C (Figure 1). With a 20mV overdrive, the maximum propagation delay is 12ns and the clock and signal rate is 85MHz.