AD8638/AD8639

Rev. C | Page 14 of 20

THEORY OF OPERATION

The AD8638/AD8639 are single-supply and dual-supply, ultrahigh

precision, rail-to-rail output operational amplifiers. The typical

offset voltage of 3 μV allows the amplifiers to be easily configured

for high gains without risk of excessive output voltage errors. The

extremely small temperature drift of 30 nV/°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.

The AD8638/AD8639 achieve a high degree of precision

through a patented auto-zeroing topology. This unique

topology allows the AD8638/AD8639 to maintain low offset

voltage over a wide temperature range and over the operating

lifetime. The AD8638/AD8639 also optimize the noise and

bandwidth over previous generations of auto-zero amplifiers,

offering the lowest voltage noise of any auto-zero amplifier by

more than 50%.

Previous designs used either auto-zeroing or chopping to add

precision to the specifications of an amplifier. Auto-zeroing

results in low noise energy at the auto-zeroing frequency, at the

expense of higher low frequency noise due to aliasing of wide-

band noise into the auto-zeroed frequency band. Chopping

results in lower low frequency noise at the expense of larger

noise energy at the chopping frequency. The AD8638/AD8639

use both auto-zeroing and chopping in a patented ping-pong

arrangement to obtain lower low frequency noise together with

lower energy at the chopping and auto-zeroing frequencies,

maximizing the SNR for the majority of applications without

the need for additional filtering. The relatively high clock

frequency of 15 kHz simplifies filter requirements for a wide,

useful, noise-free bandwidth.

The AD8638 is among the few auto-zero amplifiers offered in

the 5-lead SOT-23 package. This provides significant improve-

ment over the ac parameters of previous auto-zero amplifiers. The

AD8638/AD8639 have low noise over a relatively wide bandwidth

(0 Hz to 10 kHz) and can be used where the highest dc precision is

required. In systems with signal bandwidths ranging from 5 kHz

to 10 kHz, the AD8638/AD8639 provide true 16-bit accuracy,

making this device the best choice for very high resolution

systems.

1/f NOISE

1/f noise, also known as pink noise, is a major contributor to

errors in dc-coupled measurements. This 1/f noise error term

can be in the range of several microvolts or more and, when

amplified by the closed-loop gain of the circuit, can show up

as a large output signal. For example, when an amplifier with

5 μV p-p 1/f noise is configured for a gain of 1000, its output has

5 mV of error due to the 1/f noise. However, the AD8638/AD8639

eliminate 1/f noise internally and thus significantly reduce

output errors.

The internal elimination of 1/f noise is accomplished as follows:

1/f noise appears as a slowly varying offset to AD8638/AD8639

inputs. Auto-zeroing corrects any dc or low frequency offset.

Therefore, the 1/f noise component is essentially removed,

leaving the AD8638/AD8639 free of 1/f noise.

INPUT VOLTAGE RANGE

The AD8638/AD8639 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 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 maximum.

OUTPUT PHASE REVERSAL

Output phase reversal occurs in some amplifiers when the input

common-mode voltage range is exceeded. As common-mode

voltage is moved outside the common-mode range, the outputs

of these amplifiers can suddenly jump in the opposite direction

to the supply rail. This is the result of the differential input pair

shutting down, causing a radical shifting of internal voltages

that results in the erratic output behavior.

The AD8638/AD8639 amplifiers have been carefully designed

to prevent any output phase reversal if both inputs are main-

tained within the specified input voltage range. If one or both

inputs exceed the input voltage range but remain within the

supply rails, an internal loop opens and the output varies.

Therefore, the inputs should always be less than at least 2 V

below the positive supply.

OVERLOAD RECOVERY TIME

Many auto-zero amplifiers are plagued by a long overload recovery

time, often in milliseconds, due to the complicated settling

behavior of the internal nulling loops after saturation of the

outputs. The AD8638/AD8639 are designed so that internal

settling occurs within two clock cycles after output saturation

happens. This results in a much shorter recovery time, less than

50 μs, when compared to other auto-zero amplifiers. The wide

bandwidth of the AD8638/AD8639 enhances performance when

the parts are used to drive loads that inject transients into the

outputs. This is a common situation when an amplifier is used

to drive the input of switched capacitor ADCs.