AD7894 Analog Devices, AD7894 Datasheet - Page 10

• Manufacturer Part Number: AD7894
• Manufacturer: Analog Devices

Specifications of AD7894

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 200kSPS
• Interface: Ser
• Analog Input Type: SE-Bip,SE-Uni
• Ain Range: Bip (Vref),Bip (Vref) x 4,Bip 10V,Bip 2.5V,Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: SOIC

AD7894
AD7894 to ADSP-2101/5 Interface
An interface circuit between the AD7894 and the ADSP-2101/5
DSP processor is shown in Figure 8. In the interface shown, the
RFS1 output from the ADSP-2101/5s SPORT1 serial port is
used to gate the serial clock (SCLK1) of the ADSP-2101/5
before it is applied to the SCLK input of the AD7894. The
RFS1 output is configured for active high operation. The BUSY
line from the AD7894 is connected to the IRQ2 line of the
ADSP-2101/5 so that at the end of conversion an interrupt is
generated telling the ADSP-2101/5 to initiate a read operation.
The interface ensures a noncontinuous clock for the AD7894’s
serial clock input, with only 16 serial clock pulses provided and
the serial clock line of the AD7894 remaining low between data
transfers. The SDATA line from the AD7894 is connected to
the DR1 line of the ADSP-2101/5’s serial port.
The timing relationship between the SCLK1 and RFS1 outputs
of the ADSP-2101/5 are such that the delay between the rising
edge of the SCLK1 and the rising edge of an active high RFS1
is up to 30 ns. There is also a requirement that data must be set
up 10 ns prior to the falling edge of the SCLK1 to be read cor-
rectly by the ADSP-2101/5. The data access time for the AD7894
is 60 ns (A, B versions) from the rising edge of its SCLK input.
Assuming a 10 ns propagation delay through the external AND
gate, the high time of the SCLK1 output of the ADSP-2105
must be
that the serial clock frequency with which the interface of Figure
8 can work is limited to 4.5 MHz.
Another alternative scheme is to configure the ADSP-2101/5
such that it accepts an external noncontinuous serial clock. In
this case, an external noncontinuous serial clock is provided that
drives the serial clock inputs of both the ADSP-2101/5 and the
AD7894. In this scheme, the serial clock frequency is limited to
the processor’s cycle rate, up to a maximum of 13.8 MHz.
AD7894 to DSP56002/L002 Interface
Figure 9 shows an interface circuit between the AD7894 and the
DSP56002/L002 DSP processor. The DSP56002/L002 is
configured for normal-mode asynchronous operation with gated
clock. It is also set up for a 16-bit word with SCK as gated
clock output. In this mode, the DSP56002/L002 provides 16
serial clock pulses to the AD7894 in a serial read operation.
The DSP56002/L002 assumes valid data on the first falling
edge of SCK so the interface is simply three-wire as shown in
Figure 9.
Figure 8. AD7894 to ADSP-2101/5 Interface
ADSP-2101/5
(30 + 60 + 10 + 10) ns, i.e., 110 ns. This means
SCLK1
RFS1
IRQ2
DR1
BUSY
SCLK
SDATA
AD7894
–10–
The BUSY line from the AD7894 is connected to the MODA/
IRQA input of the DSP56002/L002 so that an interrupt will be
generated at the end of conversion. This ensures that the read
operation will take place after conversion is finished.
AD7894 PERFORMANCE
Linearity
The linearity of the AD7894 is determined by the on-chip
14-bit D/A converter. This is a segmented DAC which is laser
trimmed for 14-bit integral linearity and differential linearity.
Typical relative accuracy numbers for the part are 1/2 LSB
while the typical DNL errors are 1/3 LSB.
Noise
In an A/D converter, noise exhibits itself as code uncertainty in
dc applications and as the noise floor (in an FFT, for example)
in ac applications. In a sampling A/D converter like the AD7894,
all information about the analog input appears in the baseband
from dc to 1/2 the sampling frequency. The input bandwidth of
the track/hold exceeds the Nyquist bandwidth, so an antialiasing
filter should be used to remove unwanted signals above f
the input signal in applications where such signals exist.
Figure 10 shows a histogram plot for 8192 conversions of a dc
input using the AD7894. The analog input was set at the center
of a code transition. It can be seen that almost all the codes
appear in the one output bin indicating very good noise perfor-
mance from the ADC.
Figure 10. Histogram of 8192 Conversions of a DC Input
Figure 9. AD7894 to DSP56002/L002 Interface
6000
5000
4000
3000
2000
1000
DSP56002/L002
0
MODA/IRQA
97
SCK
SDR
98
99
ADC CODE
100
101
BUSY
SCLK
SDATA
AD7894
102
103
REV. 0
S
/2 in