Rainbow Electronics MAX1205 User Manual

Page 9

external amplifiers to compensate for any resistive drop
on these lines, internal or external to the chip. Ensure a
correct reference voltage by using proper Kelvin con-
nections at the sense pins.

Common-Mode Voltage

The switched capacitor input circuit at the analog input
allows signals between AGND and the analog power
supply. Since the common-mode voltage has a strong
influence on the performance of the ADC, the best
results are obtained by choosing V

to be at half the

difference between the reference voltages V

RFP

and

RFN

. Achieve this by using a resistive divider between

the two reference potentials. Figure 4 shows a typical
driving circuit for good dynamic performance.

Analog Signal Conditioning

For single-ended inputs the negative analog input pin
(INN) is connected to the common-mode voltage pin
(CM), and the positive analog input pin (INP) is con-
nected to the input.

To take full advantage of the ADC’s superior AC perfor-
mance up to the Nyquist frequency, drive the chip with
differential signals. In communication systems, the sig-
nals may inherently be available in differential mode.
Medical and/or other applications may only provide sin-

gle-ended inputs. In this case, convert the single-
ended signals into differential ones by using the circuit
recommended in Figure 5. Use low-noise, wideband
amplifiers such as the MAX4108 to maintain the signal
purity over the full-power bandwidth of the MAX1205
input.

Lowpass or bandpass signals may be required to
improve the signal-to-noise-and-distortion ratio of the
incoming signal. For low-frequency signals (<100kHz),
active filters may be used. For higher frequencies, pas-
sive filters are more convenient.

Single-Ended to Differential

Conversion Using Transformers

An alternative single-ended to differential-ended con-
version method is a balun transformer such as the
CTX03-13675 from Coiltronics. An important benefit of
these transformers is their ability to level-shift single-
ended signals referred to ground on the primary side to
optimum common-mode voltages on the secondary
side. At frequencies below 20kHz, the transformer core
begins to saturate, causing odd-order harmonics.

Clock Source Requirements

Pipelined ADCs typically need a 50% duty cycle clock.
To avoid this constraint, the MAX1205 provides a

MAX1205

+5V Single-Supply, 1Msps, 14-Bit

Self-Calibrating ADC

_______________________________________________________________________________________

RFP = 4.096V

RFN = 0V

MAX410

CHIP BOUNDARY

RFNS

RFNF

RFPS

RFPF

MAX410

Figure 4. Drive Circuit for the Reference Pins and the Common-
Mode Pin

MAX4108

INP

INN

V-

V+

V-

MAX4108

Figure 5. A simple circuit generates differential signals from a
single-ended input referred to analog ground. The common-
mode voltage at INP and INN is the same as CM.