ADA4627-1_10 AD [Analog Devices], ADA4627-1_10 Datasheet - Page 14

• Manufacturer Part Number: ADA4627-1_10
• Description: 30 V, High Speed, Low Noise, Low Bias Current, JFET Operational Amplifier
• Manufacturer: AD [Analog Devices]

ADA4627-1/ADA4637-1
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.
With a 100 kΩ potentiometer, the adjustment range is
more than ±11 mV. However, the V
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
Figure 47. Standard Offset Null Circuit
+V
7
2
3
ADA4627-1
S
–V
1
4
S
100kΩ
5
OS
6
temperature drift
Rev. C | Page 14 of 20
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.
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 part, combined with the thermal resistance
of the package, results in the junction temperature increasing
up 20 degrees to 30 degrees Celsius 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
calculations, and selection tables for more than three dozen low
noise, op amp families.
Figure 48. Alternate Offset Null Circuit for Inverting Stage
V
IN
+
–
R
IN
200Ω
499kΩ
2
3
AN-940
ADA4627-1
499kΩ
0.1µF
for a discussion of noise,
R
F
+V
–V
S
S
6
100kΩ
V
OUT
+
–