ADA4627-1

Rev. A | Page 12 of 16

THEORY OF OPERATION

The ADA4627-1 is a high speed, unity gain stable amplifier with

excellent dc characteristics. The typical offset voltage of 70 μV

allows the amplifier to be easily configured for high gains

without the risk of excessive output voltage errors. The small

temperature drift of 2 μV/°C ensures a minimum offset voltage

error over the entire temperature range of −40°C to +125°C,

making the amplifier ideal for a variety of sensitive measure-

ment applications in harsh operating environments.

INPUT VOLTAGE RANGE

The ADA4627-1 is not a rail-to-rail input amplifier, thus, care

is required to ensure that both inputs do not exceed the input

voltage range. Under normal negative feedback operating

conditions, the amplifier corrects its output to ensure that

the two inputs are at the same voltage. However, if either input

exceeds the input voltage range, the loop opens and large

currents begin to flow through the ESD protection diodes in

the amplifier.

These diodes are connected between the inputs and each supply

rail to protect the input transistors against an electrostatic discharge

event, and they are normally reverse-biased. However, if the

input voltage exceeds the supply voltage, these ESD diodes can

become forward-biased. Without current limiting, excessive

amounts of current may flow through these diodes, causing

permanent damage to the device. If inputs are subject to over-

voltage, insert appropriate series resistors to limit the diode

current to less than 5 mA.

INPUT OFFSET VOLTAGE ADJUST RANGE

The ADA4627-1 SOIC package has offset adjust pins for

compatibility with some existing designs. The recommended

offset nulling circuit is shown in Figure 37.

2

3

6

7

4

ADA4627-1

100kΩ

1

5

Figure 37. Standard Offset Null Circuit

With a 100 kΩ potentiometer, the adjustment range is

increases by several μV/°C for every millivolt of offset adjust.

The ADA4627-1 has matching thin film resistors that are laser

trimmed at two temperatures to minimize both offset voltage

and offset voltage drift. The offset voltage at room temperature

is less than 0.5 mV, and the offset voltage drift is only a few

μV/ºC or less, therefore, it is not recommended to use the offset

adjust pins, especially for offset adjust of a complete signal

chain. Signal chain offset can be addressed with an auto-

zero amplifier used to form a composite amplifier, or if the

ADA4627-1 is at an inverting amplifier stage, it can be modified

easily to create a summing amplifier where a potentiometer can

be added (see Figure 38). The LFCSP package does not have

offset adjust pins.

2

3

6

ADA4627-1

+

–

+

–

200Ω

100kΩ

499kΩ

499kΩ

0.1µF

Figure 38. Alternate Offset Null Circuit for Inverting Stage

INPUT BIAS CURRENT

Because the ADA4627-1 has a JFET input stage, the input bias

current, due to the reverse-biased junction, has a leakage

current that approximately doubles every 10°C. The power

dissipation of the part, combined with the thermal resistance of

the package, results in the junction temperature increasing 20 to

30 degrees Centigrade above ambient. This parameter is tested

with high speed ATE equipment, which does not result in the

die temperature reaching equilibrium. This is correlated with

bench measurements to match the guaranteed maximum at

room temperature in Table 2.

The input current can be reduced by keeping the temperature as

low as possible and using a light load on the output.

NOISE CONSIDERATIONS

The JFET input stage offers very low input voltage noise and

input current noise. The thermal noise of a 1 kΩ resistor at

room temperature is 4 nV/√Hz, thus low values of resistance

should be used for dc-coupled inverting and noninverting

amplifier configurations. In the case of transimpedance

amplifiers (TIAs), current noise is more important.

The ADA4627-1 is an excellent choice for both of these appli-

cations. Analog Devices offers a wide variety of low voltage

noise and low current noise op amps in a variety of processes

optimized for different supply voltage ranges. Refer to

Application Note AN-940 for a complete discussion of noise,

calculations, and selection tables for more than three dozen

low noise, op amp families.

THD + N MEASUREMENTS

Total harmonic distortion plus noise (THD + N) is usually

measured with an audio analyzer from Audio Precision, Inc.

The analyzer consists of a low distortion oscillator that is swept

from the starting frequency to the ending frequency. The