Evaluates: MAX1011
MAX1011 Evaluation Kit
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2.35V) if DC coupled, or to ground if AC coupling is
used. If the low-gain (500mVp-p) range is selected, the
input-drive requirements are twice those listed in Table
3. If the high-gain (125mVp-p) range is selected, the
input-drive requirements are half those listed in Table 3.
Offset-Correction Amplifier
The offset-correction amplifier included on the
MAX1011 is usually enabled in a typical AC-coupled
application circuit. For DC-coupled applications, the
amplifier must be disabled by installing shorting blocks
on jumpers JU3 and JU4. These jumpers short device
pins OCC+ (pin 2) and OCC- (pin 3) to ground and dis-
able the amplifier. The MAX1011 EV kit is configured
with the offset-correction amplifier enabled (jumpers
open) and AC-coupled analog inputs.
Voltage-Controlled-Oscillator Operation
The EV kit includes a voltage-controlled-oscillator
(VCO) circuit to set the analog-to-digital converter
(ADC) sampling rate using an external resonant tank
and a varactor diode. A voltage applied to the VTUNE
pad changes the varactor diode’s capacitance to
adjust the tank’s resonant frequency, which sets the
oscillator’s sampling frequency. VTUNE voltage can be
varied from 0V to a maximum of 8V.
The EV kit is designed so that a nominal VTUNE control
voltage of about 3.7V sets the ADC sampling rate to
90Msps. The VTUNE control voltage should be well fil-
tered, as any noise on the supply contributes to jitter in
the internal oscillator and degrades the converter’s
dynamic performance. Figure 1 shows the VTUNE
control-voltage typical frequency-adjustment range for
the MAX1011 EV kit (for VCO mode, refer to schematic
in Figure 2).
External Clock Operation
The MAX1011 EV kit can be converted to drive the ADC
from an external clock source. This involves removing
the external resonator components from the VCO circuit
and adding a few new components. Table 4 lists the EV
kit changes required to convert the board to accept an
external clock source. The resulting schematic is
shown in Figure 3.
The new 49.9Ω value of R3 shown in Figure 3 provides
proper termination for a 50Ω external signal generator.
AC-coupling capacitor C7 couples the external clock
signal to the MAX1011 oscillator circuitry at TNK+ (pin
7). R2 and C8 ensure that the impedance at both ports
of the oscillator is balanced. After all modifications are
complete, connect an external clock source to the BNC
connector on the EV kit marked CLOCK OVERDRIVE
(J2). The recommended clock amplitude is 1Vp-p; how-
ever, the ADC operates correctly with as little as
300mVp-p or up to 1.25Vp-p on CLOCK OVERDRIVE.
The external clock source should have low-phase noise
for best dynamic performance. A low-phase-noise
sine-wave oscillator serves this purpose well. A square-
wave clock source is not necessary to drive the
MAX1011. The device contains sufficient gain to ampli-
fy even a low-level-input sine wave to drive the ADC
comparators, while ensuring excellent dynamic perfor-
mance.
