AD7339 Analog Devices, AD7339 Datasheet - Page 10

• Manufacturer Part Number: AD7339
• Description: 8-Bit I/O Port
• Manufacturer: Analog Devices

Specifications of AD7339

• Resolution (bits): 8bit
• # Chan: 1
• Sample Rate: 2MSPS
• Interface: Par
• Analog Input Type: SE-Uni
• Ain Range: 2 V p-p
• Adc Architecture: SAR
• Pkg Type: QFP

The 4-bit offset nulling feature has a LSB size of 7.6 mV; thereby,
allowing the user to vary the DAC output by 115 mV.
D9 D8 D7 D6 D5
Address
The DACs use offset binary coding with 1 LSB = FS/256 =
2.8/256 = 10.94 mV. Table II shows the ideal input code to
output voltage relationship.
Serial DACs
The AD7339 has two serial DACs on board. The serial DACs
have an architecture similar to the parallel DACs. The 8-bit
digital word to each DAC is serially loaded. The serial DACs
have a common serial port. To distinguish between the two
DACs, 10-bit bursts are transferred to the DACs, the two MSBs
identifying the DAC to which the 8-bit word is to be loaded.
Table III shows the truth table for the two MSBs.
The serial word is loaded into the serial register using SDATA
and SCLK. SCLK is a gated clock of nominal value 256 kHz,
which should be active only when the 10-bit word is being
loaded into the register; i.e., SCLK should consist of 10 pulses.
If SCLK is continuous, or if it consists of more than 10 pulses,
the data shifted into the serial register will be shifted out of the
serial register so the register will not contain valid data. When
the serial register is not being written to, SCLK should idle low.
The serial data bits are read into the serial register on the rising
edge of SCLK, the two MSBs of the word identifying the DAC
to which the word is being written, and the eight LSBs of the
10-bit word containing the 8-bit word to be converted, the 8-bit
word being transferred MSB first. SDATA idles low.
AD7339
D9
0
0
1
1
Table I. Writing to the Parallel DACs Offset Registers
MSB
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 1
0 1 1 1 1 1 1 1
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 1
1 1 1 1 1 1 1 0
1 1 1 1 1 1 1 1
*These are the nominal output voltages with V
X
Table II. Ideal Input/Output Code Table
Table III. Serial DACs Truth Table
D8
0
1
0
1
LSB
X
DAC Latch Contents
Factory/
User Offset
DAC to be Written to
DAC A Offset Register Is Loaded
DAC 1 Register Is Loaded
DAC 0 Register Is Loaded
DAC B Offset Register Is Loaded
Analog Output, V
–1.4 V
–1.38906 V
–0.01094 V
0 V
+0.01094 V
+1.37812 V
+1.38906 V
D4
Decr/
Incr
OUT
D3 D2 D1 D0
Data Word
= 1.4 V.
OUT
*
–10–
The 8-bit word is loaded into the DAC from the register using
LATCH. Data is loaded into the DACs on the falling edge of
LATCH. When the D-to-A conversion is performed, the analog
output is altered accordingly. The analog output will remain
valid until the next falling edge of LATCH, at which stage the
next digital word in the register is converted. LATCH is nor-
mally low, the input being pulsed to load the DACs, the DACs
being loaded on the falling edge of LATCH.
The analog output is available on the SDAC0S/SDAC1S pin.
Each DAC has an analog output of 0.2 V to AVDD – 0.247 V,
an input of 00H generating an analog output of 0.2 V while a
digital input of FFH produces an analog output of AVDD –
0.247 V, i.e., the serial DACs use straight binary coding. The
analog output is generated by the on board reference. There-
fore, when AVDD is greater than 5.247 V, V
when the digital word equals all 1s. However, when AVDD is
less than 5.247 V, the output is limited to 0.247 V below AVDD
as the amplifier clips the output.
The output from the current source is converted to a voltage
using an operational amplifier. The amplifier is configured to
gain the signal by two; however, the gain of the amplifier can be
adjusted by tying a resistor between SDAC0F/SDAC1F and
SDAC0S/SDAC1S. The resistors on board the AD7339 have a
value of 20 k .
Power-Down
Each section of the AD7339 can be individually powered down.
The ADC, parallel DACs and serial DACs have individual
power-down pins, which allows each section to be powered
down when it is not being used, thus minimizing the current
consumption of the AD7339.
Pin ADCPDB is used to place the ADC in sleep mode. When
this pin is taken low, the ADC is powered down. For normal
operation, ADCPDB is high.
When the parallel DACs are not being used, they can be placed
in power-down mode using DACPDB. When DACPDB is low,
both DACs are powered down. The reference outputs VREFA
and VREFB are also powered down. During power-down, the
analog outputs DACA and DACB, as well as the reference out-
puts, are pulled down to ground. When the DACs are powered up,
the analog outputs settle to the bias voltage VREFA/VREFB.
The serial DACs are powered down using SDACPDB. When
this pin is tied low, the serial DACs are placed in sleep mode.
When a converter is powered up, 100 s are required for the
analog and digital circuitry to settle. Conversions can commence
when the circuitry has settled.
The reference on board the AD7339 is permanently powered
up. While the outputs VREFA and VREFB can be powered
down, the reference voltage, which is available on pin VREF, is
always available.
OUT
= 2 VREF
REV. 0