Rainbow Electronics MAX2036 - Manual (Page 21)

CW Mixer Output Summation
The maximum differential current output is typically
3mA
P-P
and the mixer output compliance voltage
ranges from 4.75V to 12V per mixer channel. The mixer
common-mode current in each of the differential mixer
outputs is typically 3.25mA. The total summed current
would equal N x 3.25mA in each of the 115Ω load resis-
tors (where N = number of channels). In this case, the
quiescent output voltage at +V
SUM
and -V
SUM
outputs
would be +11V - (N x 3.25mA x 115) = +11 - (8 x
3.25mA x 115) = 8.05V. The voltage swing at each out-
put, with one channel driven at max output current (dif-
ferential 3mA
P-P
) while the other channels are not
driven, would be 1.5mA
P-P
x 115Ω or 174mV
P-P
and the
differential voltage would be 348mV
P-P
. The voltage
compliance range is defined as the valid range for
+V
SUM and -V SUM in this example.
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 (pins 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 (0.1%) resistor from EXT_RES (pin
38) to ground.
Analog Input and Output Coupling
In typical applications, the MAX2036 is being driven
from a low-noise amplifier (such as the MAX2034) and
the VGA is typically driving a discrete differential anti-
alias filter into an ADC (such as the MAX1436 octal
ADC). The differential input impedance of the MAX2036
is typically 240Ω. The differential outputs of the VGA
are capable of driving a differential load capacitance to
GND at each of the VGA differential outputs of 60pF,
and differential capacitance across the VGA outputs is
10pF, R
L
= 1kΩ. The differential outputs have a com-
mon-mode bias of approximately 3.75V. AC-couple
these differential outputs if the next stage has a differ-
ent 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 repre-
sents reflections from blood. The latter reflections are
typically 25dB lower in magnitude, and hence the mea-
surement is defined with one input tone 25dB lower than
the other. The IMD3 product of interest (f
1 - (f 2 - f 1 )) pre-
sents itself as an undesired Doppler error signal in ultra-
sound applications. See Figure 6.
PCB Layout
The pin configuration of the MAX2036 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 MAX2036’s TQFP-EP
package provides a low thermal-resistance path to the
die. It is important that the PCB on which the MAX2036
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.
MAX2036
Ultrasound VGA Integrated
with CW Octal Mixer
______________________________________________________________________________________ 21
-25dB
ULTRASOUND
IMD3
f
1
- (f
2
- f
1
) f
2
+ (f
2
- f
1
) f
1
f
2
Figure 6. Ultrasound IMD3 Measurement Technique