AD7824 Analog Devices, AD7824 Datasheet - Page 8

• Manufacturer Part Number: AD7824
• Description: LC2MOS High Speed 4- & 8-Channel 8-Bit ADCs
• Manufacturer: Analog Devices

AD7824/AD7828
UNIPOLAR OPERATION
The analog input range for any channel of the AD7824/ AD7828
is 0 V to 5 V as shown in the unipolar operational diagram of Fig-
ure 10. Figure 11 shows the designed code transitions which
occur midway between successive integer LSB values (i.e.,
1/2 LSB, 3/2 LSB, 5/2 LSB, FS 3/2 LSBs). The output code is
Natural Binary with 1 LSB = FS/256 = (5/256) V = 19.5 mV.
BIPOLAR OPERATION
The circuit of Figure 12 is designed for bipolar operation. An
AD544 op-amp conditions the signal input (V
positive voltages appear at AIN 1. The closed loop transfer
function of the op amp for the resistor values shown is given
below:
The analog input range is ± 4 V and the LSB size is 31.25 mV.
The output code is complementary offset binary. The ideal
input/output characteristic is shown in Figure 13.
AIN 1 = (2.5 – 0.625 V
IN
) Volts
IN
) so that only
TIMING AND CONTROL
The AD7824/AD7828 has two digital inputs for timing and
control. These are Chip Select (CS) and Read (RD). A READ
operation brings CS and RD low which starts a conversion on
the channel selected by the multiplexer address inputs (see
Table I). There are two modes of operation as outlined by the
timing diagrams of Figures 14 and 15. Mode 0 is designed for
microprocessors which can be driven into a WAIT state. A
READ operation (i.e., CS and RD are taken low) starts a con-
version and data is read when conversion is complete. Mode l
does not require microprocessor WAIT states. A READ operation
initiates a conversion and reads the previous conversion results.
A1
0
0
1
1
±
AD7824
Table I. Truth Table for Input Channel Selection
A0
0
1
0
1
A2
0
0
0
0
1
1
1
1
AD7828
A1
0
0
1
1
0
0
1
1
A0
0
1
0
1
0
1
0
1
±
Channel
AIN 1
AIN 2
AIN 3
AIN 4
AIN 5
AIN 6
AIN 7
AIN 8