AD8638ARZ Analog Devices Inc, AD8638ARZ Datasheet - Page 14

• Manufacturer Part Number: AD8638ARZ
• Description: IC OPAMP CHOPPER R-R 37MA 8SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD8638ARZ

• Slew Rate: 2 V/µs
• Amplifier Type: Chopper (Zero-Drift)
• Number Of Circuits: 1
• Output Type: Rail-to-Rail
• Gain Bandwidth Product: 1.5MHz
• Current - Input Bias: 1pA
• Voltage - Input Offset: 3µV
• Current - Supply: 1.25mA
• Current - Output / Channel: 37mA
• Voltage - Supply, Single/dual (±): 5 V ~ 16 V, ±2.5 V ~ 8 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Op Amp Type: Low Offset Voltage
• No. Of Amplifiers: 1
• Bandwidth: 1.5MHz
• Supply Voltage Range: 5V To 16V
• Amplifier Case Style: SOIC
• No. Of Pins: 8
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

AD8638
THEORY OF OPERATION
The AD8638 is a single-supply, ultrahigh precision, rail-to-rail
output operational amplifier. The typical offset voltage of less
than 1 μV allows the amplifier 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 amplifier ideal for a variety of sensitive
measurement applications in harsh operating environments.
The AD8638 achieves a high degree of precision through a
patented auto-zeroing topology. This unique topology allows
the AD8638 to maintain low offset voltage over a wide temper-
ature range and over the operating lifetime. The AD8638 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 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 family
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 (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 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 ranging from 5 kHz to 10 kHz,
the AD8638 provides 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 μV 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 eliminates 1/f
noise internally and thus significantly reduces output errors.
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The internal elimination of 1/f noise is accomplished as follows:
1/f noise appears as a slowly varying offset to AD8638 inputs.
Auto-zeroing corrects any dc or low frequency offset. Therefore,
the 1/f noise component is essentially removed, leaving the
AD8638 free of 1/f noise.
INPUT VOLTAGE RANGE
The AD8638 is not a rail-to-rail input amplifier, therefore, care
is required to ensure that both inputs do not exceed the input
voltage range. Under normal negative feedback operating condi-
tions, 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 amplifier has been carefully designed to prevent
any output phase reversal if both inputs are maintained 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 two volts below the positive supply.
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
AD8638 is 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
enhances performance when the parts are used to drive loads
that inject transients into the outputs. This is a common situa-
tion when an amplifier is used to drive the input of switched
capacitor ADCs.