ADA4530-1ARZ-R7 datasheet(30/52 Pages) AD

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Data Sheet

ADA4530-1

APPLICATIONS INFORMATION

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Rev. C | 30 of 52

The ADA4530-1 is a single, electrometer grade, complementary

metal-oxide semiconductor (CMOS) operational amplifier with fem-

toampere input bias current and ultralow offset voltage. It operates

over a wide supply voltage range of 4.5 V (or ±2.25 V dual supply)

to 16 V (or ±8 V dual supply). It is a single-supply amplifier; its input

voltage range includes the lower supply rail and has a rail-to-rail

output. The ADA4530-1 also achieves a low offset voltage of

±40 µV maximum and offset voltage drift of ±0.5 µV/°C maximum.

The ADA4530-1 has ultralow input bias currents that are produc-

tion tested at 25°C and 125°C to ensure the device meets its

performance goals in a system application. An integrated guard

buffer is provided to minimize input pin leakage in a PCB design,

minimize board component count, and enable ease of system

design. The guard buffer output pins are also strategically placed

next to the input pins to enable easy routing of the guard ring

and to prevent coupling between the inputs, power supplies, and

the output pin.

The ADA4530-1 is suited for applications requiring very low input

bias current and low offset voltage, including, but not limited to,

preamplifier applications, for a wide variety of current output trans-

ducers (such as photodiodes and photomultiplier tubes), spectrom-

etry, chromatography, and high impedance buffering for chemical

sensors.

INPUT PROTECTION

When either input of the ADA4530-1 exceeds one of the supply

rails by more than 300 mV, the input ESD diodes become forward-

biased and large amounts of current begin to flow through them.

Without current limiting, this excessive fault current causes perma-

nent damage to the device. If the inputs are expected to be subject

to overvoltage conditions, insert a resistor in series with each input

to limit the input current to 10 mA maximum. However, consider the

resistor thermal noise effect on the entire circuit.

SINGLE-SUPPLY AND RAIL-TO-RAIL OUTPUT

The ADA4530-1 is a single-supply amplifier with an input voltage

range (IVR) from V− to V+ − 1.5 V. The amplifier has a small

keep alive input stage that allows it to function properly when the

input common-mode voltage is greater than the specified IVR. This

feature allows the ADA4530-1 to start up and recover quickly in

certain types of circuits where the IVR is violated at power-up. The

ac and dc performance of this keep alive stage is poor; do not rely

upon this keep alive stage for normal use.

Figure 100 shows the input and output waveforms of the

ADA4530-1 configured as a unity-gain buffer with a supply voltage

of ±8 V. The output tracks the input voltage over the entire range

until the output voltage is clamped at its maximum output swing.

The amplifier still operates even when the signal is outside the

specified input voltage range (−8 V ≤ IVR ≤ +6.5 V); this is due

to the keep alive stage. Additionally, the amplifier output does not

phase reverse. It is not recommended to apply an input voltage that

is outside of the input voltage range.

Figure 100. No Phase Reversal

CAPACITIVE LOAD STABILITY

The ADA4530-1 can safely drive capacitive loads of up to 250 pF

in any configuration. Driving larger capacitive loads than specified

can cause excessive overshoot, ringing, or oscillation. A heavy

capacitive load reduces phase margin and causes the amplifier

frequency response to peak. Peaking corresponds to overshooting

or ringing in the time domain. Therefore, it is recommended that

external compensation be used if the ADA4530-1 must drive a load

exceeding 250 pF. This compensation is particularly important in

the unity-gain configuration, which is the worst case for stability.

A quick and easy way to stabilize the op amp for capacitive load

drive is by adding a series resistor, RISO, between the amplifier

output terminal and the load capacitance, as shown in Figure

101. RISO isolates the amplifier output and feedback network from

the capacitive load. However, with this compensation scheme, the

output impedance as seen by the load increases, and this reduces

gain accuracy.

Figure 101. Stability Compensation with Isolating Resistor, RISO

Figure 102 shows the phase margin of the ADA4530-1 with different

values of output isolating resistors and capacitive loads. Figure

103 shows the frequency response with 1 nF capacitive load and

different isolating resistors.

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