AD1385

REV. 0

–12–

No external switches or relays are required for calibration and

all connections to the AD1385 may remain in place. The

Track/Hold is internally isolated from the analog input by ana-

log switches and used as a buffer during the calibration process.

Its signal output (Pin 27) must remain connected to the A/D

input(s) (Pin 30 and/or Pin 31, as appropriate) for successful

calibration. Hold Command Out (Pin 19) must also remain

connected to Hold Command In (Pin 22), either via direct con-

nection (synchronous sampling) or with an external OR-gate

(asynchronous sampling, Figure 19).

A calibration sequence may be initiated at any time by bringing

the CAL input (Pin 41) low. The calibration request remains

pending if a conversion is in process, and calibration begins on

the first rising clock edge after the end of that conversion. Cali-

bration begins on the first rising clock edge after CAL is as-

serted if the AD1385 is idle when calibration is requested. The

minimum pulse width for the CAL input is 20 ns. The CAL in-

put has priority over the Start Convert signal in all cases.

The CAL Status output (Pin 28) goes high as soon as calibra-

tion begins and remains high until the calibration cycle is com-

pleted. Pulsing CAL low while CAL Status is high has no

effect. A full calibration requires about 15 ms with a 10 MHz

clock and proportionately longer with slower clocks.

Calibration has no effect on the contents of the Autozero regis-

ter. The apparent zero point may shift a few LSBs as a result of

the calibration. Autozero after recalibration will provide the

greatest possible accuracy (see Autozero).

The AD1385 controller allocates 17 clock periods after the con-

clusion of a calibration cycle for Track/Hold recovery and signal

acquisition. Activity at the Start Convert input during this

interval is ignored. Figure 23 shows the timing associated with

the resumption of synchronous conversions following a calibra-

tion cycle.

Start-Convert should remain low during the calibration period

when using asynchronous sampling (Figure 19).

Figure 23. Resumption of Synchronous Conversions

Following Completion of Calibration

The CAL input may be held low indefinitely, causing repeated

calibration cycles. The AD1385 will complete the calibration in

progress when CAL goes high and will then begin normal con-

versions after the 17-clock-period delay. This simplifies the

system-level implementation of the power-up reset function.

The AD1385 requires a 5 minute warmup to reach thermal

equilibrium after power is applied, and calibration may drift

slightly during this time. Occasional recalibration will provide a

slight improvement in distortion and noise performance during

warmup.

AUTOZERO (Pin 45)

The Autozero function may be used to digitally correct internal

offsets in the Track/Hold and ADC as well as external offsets.

To use Autozero the Track/Hold input must be connected to a

zero reference prior to the zeroing conversion. This connection

is external to the AD1385 and must be provided by the user;

the resistance of this connection is not critical but should be less

than 1000

Ω. An Autozero cycle forces the AD1385’s digital

output to indicate exactly zero when its input is at the zero

point, nominally 0 V. (This assumes that the complementary

twos complement data format is used. Autozero forces the digi-

tal output to midscale when the selected data format is comple-

mentary offset binary.) Autozero operates by storing the digital

result of a zeroing conversion and subtracting it from all subse-

quent conversion results. This reduces the maximum nonsat-

urating input of the AD1385 a small amount at one end of its

range depending on the magnitude and polarity of the offset.

The Autozero feature is enabled by driving the Autozero input

(Pin 45) low before a falling edge at the Data Strobe output.

Offset data will be stored on the first rising edge of Data Strobe

after Autozero is brought high; the offset data are also available

on the AD1385’s data bus during this Data Strobe pulse. Au-

tozero operation is illustrated in Figure 24. All subsequent A/D

conversions will be digitally corrected by the offset term as long

as Autozero remains high. The offset register is cleared when

Autozero goes low and the contents of the data output registers

will revert to their uncorrected value. Figure 24 shows Autozero

timing requirements. Autozero cannot be activated until the

first conversion after power-up has been completed.

The Autozero feature may be disabled by keeping Autozero low.

Figure 24. Autozero Cycle Operation