MAX2036CCQ+T Maxim Integrated Products, MAX2036CCQ+T Datasheet - Page 19

• Manufacturer Part Number: MAX2036CCQ+T
• Description: IC VGA W/OCTAL MIXER 8CH 100TQFP
• Manufacturer: Maxim Integrated Products

Specifications of MAX2036CCQ+T

• Amplifier Type: Variable Gain
• Number Of Circuits: 8
• Output Type: Differential
• -3db Bandwidth: 2MHz
• Current - Supply: 245mA
• Current - Output / Channel: 3.75mA
• Voltage - Supply, Single/dual (±): 4.75 V ~ 5.25 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 100-TQFP Exposed Pad, 100-eTQFP, 100-HTQFP, 100-VQFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Slew Rate: -
• Gain Bandwidth Product: -
• Current - Input Bias: -
• Voltage - Input Offset: -

The mode select response time is the time that the
device takes to switch between CW and VGA modes.
One possible approach to interfacing the CW outputs to
an instrumentation amplifier used to drive an ADC is
shown in Figure 2. In this implementation, there are four
large-value (in the range of 470nF to 1µF) capacitors
between each of the CW_IOUT+, CW_IOUT-,
CW_QOUT+, CW_QOUT- outputs and the circuitry they
are driving. The output of the CW mixer usually drives
the input of an instrumentation amplifier made up of op
amps whose input impedance is set by common-mode
setting resistors.
There are clearly both a highpass corner and a lowpass
corner present in this output network. The lowpass cor-
ner is set primarily by the 115Ω mixer pullup resistors,
the series 50Ω resistors, and the shunt 0.022µF capaci-
tor. This lowpass corner is used to filter a combination
of LO leakage and upper sideband. The highpass cor-
ner, however, is of a larger concern due to the fact that
it is dominated by the combination of a 1µF DC blocking
capacitor and the pair of shunt 31.6kΩ resistors.
If drawn, the simplified dominant highpass network
would look like Figure 3.
Figure 2. Typical Example of a CW Mixer’s Output Circuit
Figure 3. Simplified Circuit of Highpass Pole
115Ω
CW_IOUT+
CW_IOUT-
115Ω
Applications Information
50Ω
______________________________________________________________________________________
Mode Select Response Time
1µF
0.022µF
31.6kΩ
31.6kΩ
1µF
1µF
31.6kΩ
Ultrasound VGA Integrated
The highpass pole in this case is at f
RC) ~ 5Hz. Note that this low highpass corner frequen-
cy is required in order to filter the downconverted clut-
ter tone, which appears at DC, but not interfere with
CWD imaging at frequencies as low as 400Hz. For
example, if one wanted to use CWD down to 400Hz,
then a good choice for the highpass pole would be at
least a decade below this (< 40Hz) as not to incur
rolloff due to pole. Remember, if the highpass pole is
set to 400Hz, the response is 3dB down at that corner
frequency. The placement of the highpass pole at 5Hz
in the above example is between the DC and 40Hz limi-
tations just discussed.
The bottom line is that any reasonably sized DC block
between the output of the mixer and the instrumentation
amplifier pose a significant time constant that slows the
mode select switching speed.
An alternative solution to the approach in Figure 2,
which enables faster mode select response time, is
shown in Figure 4.
In Figure 4, the outputs of the CWD mixers are DC-
coupled into the inputs of the instrumentation ampli-
fiers. Therefore, the op amps must be able to accom-
modate the full compliance range of the mixer outputs,
which is a maximum of +11V when the mixers are dis-
abled, down to the +5V supply of the MAX2036 when
the mixers are enabled. The op amps can be powered
from +11V for the high rail and +5V for the low rail,
requiring a 6V op amp.
Figure 4. Improved Mode Select Response Time Achieved with
DC-Coupled Input to Instrumentation Amplifier
with CW Octal Mixer
+11V
+5V
P
= 1/(2 x pi x
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