____________________Serial Interface

Initialization After Power-Up and

Starting a Conversion

When power is first applied, it takes the fully dis-

charged 4.7µF reference bypass capacitor up to 20ms

to provide adequate charge for specified accuracy.

With

SHDN not pulled low, the MAX187/MAX189 are

now ready to convert.

To start a conversion, pull

CS low. At CS’s falling edge,

the T/H enters its hold mode and a conversion is initiat-

ed. After an internally timed 8.5µs conversion period,

the end of conversion is signaled by DOUT pulling

high. Data can then be shifted out serially with the

external clock.

Using

Power consumption can be reduced significantly by

shutting down the MAX187/MAX189 between conver-

sions. This is shown in Figure 6, a plot of average sup-

ply current vs. conversion rate. Because the MAX189

uses an external reference voltage (assumed to be pre-

sent continuously), it "wakes up" from shutdown more

quickly, and therefore provides lower average supply

SHDN deasserted to the time when a conversion may

be initiated. For the MAX187, this time is 2µs. For the

MAX189, this time depends on the time in shutdown

(see Figure 7) because the external 4.7µF reference

bypass capacitor loses charge slowly during shutdown

(see the specifications for shutdown, REF Input Current

= 10µA max).

External Clock

The actual conversion does not require the external

clock. This frees the µP from the burden of running the

SAR conversion clock, and allows the conversion result

to be read back at the µP’s convenience at any clock

rate from 0MHz to 5MHz. The clock duty cycle is unre-

stricted if each clock phase is at least 100ns. Do not

run the clock while a conversion is in progress.

Timing and Control

Conversion-start and data-read operations are con-

trolled by the

CS and SCLK digital inputs. The timing

diagrams of Figures 8 and 9 outline the operation of the

serial interface.

A

CS falling edge initiates a conversion sequence: The

T/H stage holds input voltage, the ADC begins to con-

vert, and DOUT changes from high impedance to logic

low. SCLK must be kept inactive during the conversion.

An internal register stores the data when the conversion

is in progress.

End of conversion (EOC) is signaled by DOUT going

high. DOUT’s rising edge can be used as a framing

signal. SCLK shifts the data out of this register any time

after the conversion is complete. DOUT transitions on

SCLK’s falling edge. The next falling clock edge pro-

duces the MSB of the conversion at DOUT, followed by

the remaining bits. Since there are 12 data bits and one

leading high bit, at least 13 falling clock edges are

needed to shift out these bits. Extra clock pulses occur-

ring after the conversion result has been clocked out,

and prior to a rising edge of

CS, produce trailing 0s at

DOUT and have no effect on converter operation.

+5V, Low-Power, 12-Bit Serial ADCs

______________________________________________________________________________________

11

10000

1000

100

10

1

0.1

1

10

100

1000

10000

CONVERSIONS PER SECOND

100000

MAX187

MAX189*

3.0

2.5

2.0

1.5

1.0

0.5

0

0.0001

0.001

0.01

0.1

1

10

TIME IN SHUTDOWN (sec)

Figure 6. Average Supply Current vs. Conversion Rate