AD8629S

ASD0016529 Rev. C | Page 7 of 8

7.0

7.1.

Application Notes

The AD8629 is a single-supply, ultrahigh precision rail-to-rail input and output operational amplifier. The

typical offset voltage of less than 1 μV allows this amplifier to be easily configured for high gains without risk

of excessive output voltage errors. The extremely small temperature drift ensures a minimum offset voltage

error over their entire temperature range of

−55°C to +125°C, making this amplifier ideal for a variety of

sensitive measurement applications in harsh operating environments.

Functional Description

The AD8629 achieves a high degree of precision through a patented combination of auto-zeroing and

chopping. This unique topology allows the AD8629 to maintain its low offset voltage over a wide temperature

range. The AD8629 also optimizes 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 wideband 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 AD8629 uses

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 signal-to-noise ratio

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 AD8629 has 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 of from 5 kHz to 10 kHz, the AD8629

provides true 16-bit accuracy, making it the best choice for very high resolution systems.

7.2. Input Overvoltage Protection

Although the AD8629 are rail-to-rail input amplifiers, care should be taken to ensure that the potential

difference between the inputs does not exceed the supply voltage. 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 either supply rail by more than 0.3 V, 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 could flow through these diodes, causing permanent damage to the device. If inputs are

subject to overvoltage, appropriate series resistors should be inserted to limit the diode current to less than 5

mA maximum.

7.3.

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 result in the erratic output behavior.

Output Phase Reversal

The AD8629 amplifiers have been carefully designed to prevent any output phase reversal, provided that both

inputs are maintained within the supply voltages. If one or both inputs could exceed either supply voltage, a

resistor should be placed in series with the input to limit the current to less than 5 mA. This ensures that the

output does not reverse its phase.

7.4. Overload Recovery Time

Many auto-zero amplifiers are plagued by a long overload recovery time, often in ms, due to the complicated

settling behavior of the internal nulling loops after saturation of the outputs. The AD8629 has been designed

so that internal settling occurs within two clock cycles after output saturation occurs. This results in a much

shorter recovery time, less than 10 μs, when compared to other auto-zero amplifiers. The wide bandwidth of

the AD8629 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.