Applications Information
External Compensation
External compensation is required for bypassing inter-
nal biasing circuitry. Connect, as close as possible,
individual 4.7µF capacitors from each pin EXT_C1,
EXT_C2, and EXT_C3 (pin 13, 14, 15) to ground.
External Bias Resistor
An external resistor at EXT_RES is required to set the
bias for the internal biasing circuitry. Connect, as close
as possible, a 7.5kΩ resistor from EXT_RES (pin 38) to
ground.
Analog Input and Output Coupling
In typical applications, the MAX2037 is being driven
from a low-noise amplifier (such as the MAX2034) and
is typically driving a discrete differential anti-alias filter
into an ADC (such as the MAX1436 octal ADC). The
differential input impedance of the MAX2037 is typically
200Ω. The differential outputs are capable of driving a
differential load resistance of 1000Ω. The output
impedance is 100Ω differential. The differential outputs
have a common-mode bias of approximately 3.0V. AC-
couple these differential outputs if the next stage has a
different common-mode input range.
Ultrasound-Specific IMD3 Specification
Unlike typical communications specs, the two input
tones are not equal in magnitude for the ultrasound-
specific IMD3 two-tone specification. In this measure-
ment, f
1
represents reflections from tissue and f
2
represents reflections from blood. The latter reflections
are typically 25dB lower in magnitude, and hence the
measurement is defined with one input tone 25dB lower
than the other. The IMD3 product of interest (f
1
- (f
2
- f
1
))
presents itself as an undesired Doppler error signal in
ultrasound applications. See Figure 1.
Board Layout
The pin configuration of the MAX2037 is optimized to
facilitate a very compact physical layout of the device
and its associated discrete components. A typical
application for this device might incorporate several
devices in close proximity to handle multiple channels
of signal processing.
The exposed pad (EP) of the MAX2037’s TQFP-EP
package provides a low thermal-resistance path to the
die. It is important that the PCB on which the MAX2037
is mounted be designed to conduct heat from the EP.
In addition, provide the EP with a low-inductance path
to electrical ground. The EP MUST be soldered to a
ground plane on the PCB, either directly or through an
array of plated via holes.
MAX2037
Ultrasound Variable-Gain Amplifier
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