Power-Down Input (PD)

The MAX1209 has two power modes that are controlled

with the power-down digital input (PD). With PD low, the

MAX1209 is in normal operating mode. With PD high,

the MAX1209 is in power-down mode.

The power-down mode allows the MAX1209 to efficient-

ly use power by transitioning to a low-power state when

conversions are not required. Additionally, the

MAX1209 parallel output bus is high impedance in

power-down mode, allowing other devices on the bus

to be accessed.

In power-down mode, all internal circuits are off, the

analog supply current reduces to 1µA, and the digital

supply current reduces to 0.9µA. The following list

shows the state of the analog inputs and digital outputs

in power-down mode:

• INP, INN analog inputs are disconnected from the

internal input amplifier (Figure 3).

• REFOUT has approximately 17k

Ω to GND.

• REFP, COM, and REFN go high impedance with

Ω

resistor between REFP and COM, as well as an inter-

nal 4k

Ω resistor between REFN and COM.

• D11–D0, DOR, and DAV go high impedance.

• CLKP and CLKN go high impedance (Figure 5).

The wake-up time from power-down mode is dominat-

ed by the time required to charge the capacitors at

REFP, REFN, and COM. In internal reference mode and

buffered external reference mode, the wake-up time is

typically 10ms with the recommended capacitor array

(Figure 13). When operating in unbuffered external ref-

erence mode, the wake-up time is dependent on the

external reference drivers.

Applications Information

Using Transformer Coupling

In general, the MAX1209 provides better SFDR and THD

performance with fully differential input signals as

opposed to single-ended input drive. In differential input

mode, even-order harmonics are lower as both inputs are

balanced, and each of the ADC inputs only requires half

the signal swing compared to single-ended input mode.

An RF transformer (Figure 10) provides an excellent

solution to convert a single-ended input source signal

to a fully differential signal, required by the MAX1209

for optimum performance. Connecting the center tap of

shift to the input. Although a 1:1 transformer is shown, a

step-up transformer can be selected to reduce the

drive requirements. A reduced signal swing from the

input driver, such as an op amp, can also improve the

overall distortion. The configuration of Figure 10 is good

12-Bit, 80Msps, 3.3V IF-Sampling ADC

______________________________________________________________________________________

19

DIFFERENTIAL INPUT VOLTAGE (LSB)

-1

-2045

4096

1 LSB =

0+1

-2047

+2047

+2045

0x800

0x801

0x802

0x803

0x7FF

0x7FE

0x7FD

0xFFF

0x000

0x001

DIFFERENTIAL INPUT VOLTAGE (LSB)

-1

-2045

4096

1 LSB =

0+1

-2047

+2047

+2045

0x000

0x001

0x003

0x002

0x800

0x801

0x803

0x400

0xC00

0xC01

Figure 8. Gray Code Transfer Function (G/T = 1)