ADA4627-1/ADA4637-1

Data Sheet

Rev. F | Page 14 of 20

THEORY OF OPERATION

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

excellent dc characteristics. The ADA4637-1 is a decompensated

version that is stable at a gain of 5 or greater. The typical offset

voltage of 70 μV allows the amplifiers 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 amplifiers ideal for a variety of sensitive

measurement applications in harsh operating environments.

INPUT VOLTAGE RANGE

The ADA4627-1/ADA4637-1 are not rail-to-rail input

amplifiers; therefore, 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 can flow through these diodes, causing permanent

damage to the device. If inputs are subject to overvoltage, insert

appropriate series resistors to limit the diode current to less

than 5 mA.

INPUT OFFSET VOLTAGE ADJUST RANGE

The ADA4627-1/ADA4637-1 SOIC packages have offset

adjust pins for compatibility with some existing designs. The

recommended offset nulling circuit is shown in Figure 47.

Figure 47. 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/ADA4637-1 have 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/°C 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 or the ADA4637-1 is in an inverting amplifier

stage, it can be modified easily to add a potentiometer (see

Figure 48). The LFCSP package does not have offset adjust pins.

Figure 48. Alternate Offset Null Circuit for Inverting Stage

INPUT BIAS CURRENT

Because the ADA4627-1/ADA4637-1 have 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 device, combined with the thermal

resistance of the package, results in the junction temperature

increasing 20°C to 30°C 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 shown 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; therefore, 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/ADA4637-1 are an excellent choice for both of

these applications. Analog Devices offers a wide variety of low

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

processes that are optimized for different supply voltage ranges.

Refer to Application Note AN-940 for a discussion of noise,

calculations, and selection tables for more than three dozen low

noise, op amp families.

2

3

6

7

4

ADA4627-1

100kΩ

1

5

2

3

6

ADA4627-1

+

–

+

–

200Ω

100kΩ

499kΩ

499kΩ

0.1µF