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ADA4530-1

Data Sheet

Rev. A | Page 30 of 50

APPLICATIONS INFORMATION

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

operational amplifier with femtoampere 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, where 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

production tested at 25°C and 125°C to ensure that the device

meets its performance goals in a system application. An

integrated guard buffer is provided to minimize input pin

leakage in a printed circuit board (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 transducers (photodiodes, photomultiplier

tubes), spectrometry, 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 permanent 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. Note that 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, it

does not phase reverse. It is not recommended to apply an input

voltage that is outside of the input voltage range.

–2

–6

–4

–8

–10

TIME (200µs/DIV)

VSY = ±8V

VIN = ±8.3V

AV = +1

RL = 10kΩ

CL = 10pF

VIN

VOUT

Figure 100. No Phase Reversal

CAPACITIVE LOAD STABILITY

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

250 pF in any configuration. As with most amplifiers, driving

larger capacitive loads than specified may cause excessive

overshoot and ringing, or even 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.

–VSY

VIN

+VSY

VOUT

ADA4530-1

RISO

Figure 101. Stability Compensation with Isolating Resistor, RISO

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