ADA4622-1ARJZ-R2 datasheet(27/36 Pages) AD

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Data Sheet

ADA4622-1/ADA4622-2/ADA4622-4

THEORY OF OPERATION

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Rev. F | 27 of 36

The ADA4622-1/ADA4622-2/ADA4622-4 are designed for

12 nV/√Hz wideband input voltage noise density and maintain

low noise performance at low frequencies (see Figure 84). This

noise performance, along with the low input current as well as low

current noise, means that the ADA4622-1/ADA4622-2/ADA4622-4

contribute negligible noise for applications with a source resistance

greater than 10 kΩ and at signal bandwidths greater than 1 kHz.

Figure 84. Total Noise vs. Source Resistance and Frequency

Input Overvoltage Protection

The ADA4622-1/ADA4622-2/ADA4622-4 have internal protective

circuitry that allows voltages as high as 0.3 V beyond the supplies

applied at the input of either terminal without causing damage. Use

a current-limiting resistor in series with the input of the ADA4622-1/

ADA4622-2/ADA4622-4 if the input voltage exceeds 0.3 V beyond

the supply rails of the amplifiers. If the overvoltage condition per-

sists for more than a few seconds, damage to the amplifiers can

result.

For higher input voltages, determine the resistor value by

VIN−VSYRS

≤10mA

(1)

where:

VIN is the input voltage.

VSY is the voltage of either the V+ pin or the V− pin.

RS is the series resistor.

With a very low input bias current of ±1.5 nA maximum up to 125°C,

higher resistor values can be used in series with the inputs without

introducing large offset errors. A 1 kΩ series resistor allows the

ADA4622-1/ADA4622-2/ADA4622-4 to withstand 10 V of continu-

ous overvoltage and increases the noise by a negligible amount.

A 5 kΩ resistor protects the inputs from voltages as high as 25 V

beyond the supplies and adds less than 10 µV to the offset voltage

of the amplifiers.

EMI Rejection Ratio

Figure 85 shows the EMI rejection ratio (EMIRR) vs. the frequency

for the ADA4622-1/ADA4622-2/ADA4622-4.

Figure 85. EMIRR vs. Frequency

OUTPUT CHARACTERISTICS

The ADA4622-1/ADA4622-2/ADA4622-4 unique bipolar rail-to-rail

output stage swings within 10 mV of the supplies with no external

resistive load.

The approximate output saturation resistance of the ADA4622-1/

ADA4622-2/ADA4622-4 is 24 Ω, sourcing or sinking. Use the output

impedance to estimate the output saturation voltage when driving

heavier loads. As an example, when driving 5 mA, the saturation

voltage from either rail is approximately 120 mV.

If the ADA4622-1/ADA4622-2/ADA4622-4 output drives hard

against the output saturation voltage, it recovers within 1.2 µs of the

input, returning to the linear operating region of the amplifier (see

Figure 56 and Figure 59).

Capacitive Load Drive Capability

Direct capacitive loads interact with the effective output impedance

of the ADA4622-1/ADA4622-2/ADA4622-4 to form an additional

pole in the feedback loop of the amplifiers, which causes excessive

peaking on the pulse response or loss of stability. The worst

case condition is when the devices use a single 5 V supply in a

unity-gain configuration. Figure 86 shows the pulse response of the

ADA4622-1/ADA4622-2/ADA4622-4 when driving 500 pF directly.

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