HM722 datasheet(9/13 Pages) HMSEMI

9 / 13 page

Application Notes (continued)

Figure 5. Differential Amplifier

Figure 6. Instrumentation Amplifier

Typical Application Circuits

The CF and RISO serve to counteract the loss of phase

margin by feeding the high frequency component of the

output signal back to the amplifier’s inverting input, thereby

preserving phase margin in the overall feedback loop.

For no-buffer configuration, there are two others ways to

increase the phase margin: (a) by increasing the amplifier’s

gain, or (b) by placing a capacitor in parallel with the

feedback resistor to counteract the parasitic capacitance

associated with inverting node.

POWER SUPPLY LAYOUT AND BYPASS

The H

M72x family operates from either a single +2.3V to

+5.5V supply or dual ±1.15V to ±3.00V supplies. For

single-supply operation, bypass the power supply VS with a

ceramic capacitor (i.e. 0.01μF to 0.1μF) which should be

placed close (within 2mm for good high frequency

performance) to the VS pin. For dual-supply operation, both

the VS+ and the VS– supplies should be bypassed to ground

with separate 0.1μF ceramic capacitors. A bulk capacitor

(i.e. 2.2μF or larger tantalum capacitor) within 100mm to

provide large, slow currents and better performance. This

bulk capacitor can be shared with other analog parts.

Good PC board layout techniques optimize performance by

decreasing the amount of stray capacitance at the op-amp’s

inputs and output. To decrease stray capacitance, minimize

trace lengths and widths by placing external components as

close to the device as possible. Use surface-mount

components whenever possible. For the op-amp, soldering

the part to the board directly is strongly recommended. Try

to keep the high frequency big current loop area small to

minimize the EMI (electromagnetic interfacing).

GROUNDING

A ground plane layer is important for the H

M72x circuit

design. The length of the current path speed currents in an

inductive ground return will create an unwanted voltage

noise. Broad ground plane areas will reduce the parasitic

inductance.

INPUT-TO-OUTPUT COUPLING

To minimize capacitive coupling, the input and output signal

traces should not be parallel. This helps reduce unwanted

positive feedback.

Figure 4. Indirectly Driving Heavy Capacitive Load with DC

Accuracy

DIFFERENTIAL AMPLIFIER

The circuit shown in Figure 5 performs the difference

function. If the resistors ratios are equal R4/R3 =R2/R1, then:

VOUT =(Vp –Vn) × R2/R1 +VREF

INSTRUMENTATION AMPLIFIER

The H

M72x family is well suited for conditioning sensor

signals in battery-powered applications. Figure 6 shows a

two op-amp instrumentation amplifier, using the H

M72x op-

amps. The circuit works well for applications requiring

rejection of common-mode noise at higher gains. The

reference voltage (VREF) is supplied by a low-impedance

source. In single voltage supply applications, the VREF is

typically VS/2.

BUFFERED CHEMICAL SENSORS

The H

M72x family has input bias current in the pA range.

This is ideal in buffering high impedance chemical sensors,

such as pH probes. As an example, the circuit in Figure 7

eliminates expansive low-leakage cables that that is

required to connect a pH probe (general purpose

combination pH probes, e.g Corning 476540) to metering

ICs such as ADC, AFE and/or MCU. An H

M72x op-amp

)(1

)

OUT

REF





VOUT

VREF

Figure 7. Buffered pH Probe

H 72x

HM721, HM722, HM724

12MHz, High Slew Rate, RRIO CMOS Operational Amplifiers

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