AD8436 AD [Analog Devices], AD8436 Datasheet - Page 11

• Manufacturer Part Number: AD8436
• Description: Low Cost, Low Power
• Manufacturer: AD [Analog Devices]

The 16 kΩ resistor in the output converts the output current to
a dc voltage that can be connected to the output buffer or to the
circuit that follows. The output appears as a voltage source in
series with 16 kΩ. If a current output is desired, the resistor
connection to ground is left open and the output current is
applied to a subsequent circuit, such as the summing node of
a current summing amplifier. Thus, the core has both current
and voltage outputs, depending on the configuration. For a
voltage output with 0 Ω source impedance, use the output
buffer. The offset voltage of the buffer is 25 μV or 50 μV,
depending on the grade.
FET Input Buffer
Referring to Figure 1, the input resistance of the
and a voltage source input is preferred. The optional input buffer
is a wideband JFET input amplifier that minimally loads non-0 Ω
sources, such as a tapped resistor attenuator or voltage sensor.
Although the input buffer consumes only 150 μA, the supply is
pinned out and left unconnected to reduce power where needed.
Optional matched 10 kΩ input and feedback resistors are provided
on chip. Consult the Applications Information section to learn
how these resistors can be used. The 3 dB bandwidth of the input
buffer is 2.7 MHz at 10 mV rms input and approximately 1.5 MHz
at 1 V rms. The amplifier gain and bandwidth are sufficient for
applications requiring modest gain or response enhancement to
a few hundred kilohertz (kHz), if desired. Configurations of the
input buffer are discussed in the Applications Information
section.
Precision Output Buffer
The precision output buffer is a bipolar input amplifier, laser
trimmed to cancel input offset voltage errors. As with the input
buffer, the supply current is very low (<50 μA, typically), and the
power can be disconnected for power savings if the buffer is not
needed. Be sure that the noninverting input is also disconnected
from the core output (OUT) if the buffer supply pin is discon-
nected. Although the input current of the buffer is very low,
a laser-trimmed 16 kΩ resistor, connected in series with the
inverting input, offsets any self-bias offset voltage.
AD8436
is 8 kΩ,
Rev. 0 | Page 11 of 20
The output buffer can be configured as a single or two-pole low-
pass filter using circuits shown in the Applications Information
section. Residual output ripple is reduced, without affecting the
converted dc output. As the response approaches the low
frequency end of the bandwidth, the ripple rises, dependent on
the value of the averaging capacitor. Figure 26 shows the effects of
four combinations of averaging and filter capacitors. Although
the filter capacitor reduces the ripple for any given frequency, the
dc error is unaffected. Of course, a larger value averaging
capacitor can be selected, at a larger cost. The advantage of using
a low-pass filter is that a small value of filter capacitor, in
conjunction with the 16 kΩ output resistor, reduces ripple and
permits a smaller averaging capacitor, effecting a cost savings.
The recommended capacitor values for operation to 40 Hz are
10 μF for averaging and 3.3 μF for filter.
Dynamic Range
The
dynamic range. Although accuracy varies slightly more at the
extreme input values, the device still converts with no spurious
noise or dropout. Figure 24 is a plot of the rms/dc transfer function
near zero voltage. Unlike processor or other solutions, residual
errors at very low input levels can be disregarded for most
applications.
30
20
10
0
–30
AD8436
is a translinear rms-to-dc converter with exceptional
–20
SOLUTION
Figure 24. DC Transfer Function near Zero
AD8436
–10
INPUT VOLTAGE (mV DC)
∆Σ OR OTHER DIGITAL
SOLUTIONS CANNOT
WORK AT ZERO
VOLTS
0
10
20
AD8436
30