HM8091/8092/8094/8091N/8092N
350MHZ CMOS Rail-to-Rail Output Opamps
Application Note
Driving Capacitive Loads
HM809X series op amps are unity-gain stable and suitable for a wide range of general-purpose applications. The small
footprints of the HM809X series packages save space on printed circuit boards and enable the design of smaller electronic
products.
Power Supply Bypassing and Board Layout
HM809X series operates from a single 2.5V to 5.5V supply or dual ±1.25V to ±2.75V supplies. For best performance, a 0.1µF
ceramic capacitor should be placed close to the VDD pin in single supply operation. For dual supply operation, both VDD and VSS
supplies should be bypassed to ground with separate 0.1µF ceramic capacitors.
Low Supply Current
The low supply current (typical 4.2mA per channel) of HM809X series will help to maximize battery life. They are ideal for
battery powered systems
Operating Voltage
HM809X series operate under wide input supply voltage (2.5V to 5.5V). In addition, all temperature specifications apply from
-40 oC to +125 oC. Most behavior remains unchanged throughout the full operating voltage range. These guarantees ensure
operation throughout the single Li-Ion battery lifetime
Rail-to-Rail Output
Rail-to-Rail output swing provides maximum possible dynamic range at the output. This is particularly important when
operating in low supply voltages. The output voltage of HM809X series can typically swing to less than 30mV from supply rail in
light resistive loads (>1kΩ), and 120mV of supply rail in moderate resistive loads (150Ω).
Capacitive Load Tolerance
The HM809X family is optimized for bandwidth and speed, not for driving capacitive loads. Output capacitance will create a
pole in the amplifier’s feedback path, leading to excessive peaking and potential oscillation. If dealing with load capacitance is
a requirement of the application, the two strategies to consider are (1) using a small resistor in series with the amplifier’s output
and the load capacitance and (2) reducing the bandwidth of the amplifier’s feedback loop by increasing the overall noise gain.
Figure 2. shows a unity gain follower using the series resistor strategy. The resistor isolates the output from the capacitance
and, more importantly, creates a zero in the feedback path that compensates for the pole created by the output capacitance.
Figure 2. Indirectly Driving a Capacitive Load Using Isolation Resistor
The bigger the RISO resistor value, the more stable VOUT will be. However, if there is a resistive load RL in parallel with the
capacitive load, a voltage divider (proportional to RISO/RL) is formed, this will result in a gain error.
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