AD8638ARJZ-R2 Analog Devices Inc, AD8638ARJZ-R2 Datasheet - Page 14

• Manufacturer Part Number: AD8638ARJZ-R2
• Description: IC OPAMP CHOPPER R-R SOT23-5
• Manufacturer: Analog Devices Inc

Specifications of AD8638ARJZ-R2

• 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: SOT-23-5, SC-74A, SOT-25
• Op Amp Type: Wideband
• No. Of Amplifiers: 1
• Bandwidth: 1.5MHz
• Supply Voltage Range: 5V To 16V
• Amplifier Case Style: SOT-23
• No. Of Pins: 5
• 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/AD8639
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.
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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/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 10 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.