Datasheet

200 ohm
7 pF3 pF
20M ohm
3 pF
5 pF
80 ohm
Ch0 assumed to be on
Chn assumed to be off
Ch0
Chn
MXO
AINP
W
W
W
ADS7950, ADS7951, ADS7952, ADS7953
ADS7954, ADS7955, ADS7956, ADS7957
ADS7958, ADS7959, ADS7960, ADS7961
SLAS605A –JUNE 2008–REVISED JANUARY 2010 www.ti.com
Figure 59. ADC and Mux Equivalent Circuit
REFERENCE
The ADS79XX can operate with an external 2.5V ± 10mV reference. A clean, low noise, well-decoupled
reference voltage on the REF pin is required to ensure good performance of the converter. A low noise band-gap
reference like the REF5025 can be used to drive this pin. A 10-mF ceramic decoupling capacitor is required
between the REF and GND pins of the converter. The capacitor should be placed as close as possible to the
pins of the device.
POWER SAVING
The ADS79XX devices offer a power-down feature to save power when not in use. There are two ways to
powerdown the device. It can be powered down by writing DI05 = 1 in the Mode Control register (refer to
Table 1, Table 2 and Table 5); in this case the device powers down on the 16th falling edge of SCLK in the next
data frame. Another way to powerdown the device is through GPIO. GPIO3 can act as a PD input (refer to
Table 10, for assigning this functionality to GPIO3). This is an asynchronous and active low input. The device
powers down instantaneously after GPIO3 (PD) = 0. The device will powerup again on the CS falling edge while
DI05 = 0 in the Mode Control register and GPIO3 (PD) = 1.
DIGITAL OUTPUT
As discussed previously in the Device Operation section, the digital output of the ADS79XX devices is SPI
compatible. The following table lists the output codes corresponding to various analog input voltages.
Table 11. Ideal Input Voltages and Output Codes for 12-Bit Devices (ADS7950/51/52/53)
DESCRIPTION ANALOG VALUE DIGITAL OUTPUT
Full scale range Range 1 → V
ref
Range 2 → 2×V
ref
STRAIGHT BINARY
Least significant bit (LSB) V
ref
/4096 2V
ref
/4096 BINARY CODE HEX CODE
Full scale V
ref
– 1 LSB 2V
ref
– 1 LSB 1111 1111 1111 FFF
Midscale V
ref
/2 V
ref
1000 0000 0000 800
Midscale – 1 LSB V
ref
/2 – 1 LSB V
ref
– 1 LSB 0111 1111 1111 7FF
Zero 0 V 0 V 0000 0000 0000 000
Table 12. Ideal Input Voltages and Output Codes for 10-Bit Devices (ADS7954/55/56/57)
DESCRIPTION ANALOG VALUE DIGITAL OUTPUT
Full scale range Range 1 → V
ref
Range 2 → 2×V
ref
STRAIGHT BINARY
Least significant bit (LSB) V
ref
/1024 2V
ref
/1024 BINARY CODE HEX CODE
Full scale V
ref
– 1 LSB 2V
ref
– 1 LSB 11 1111 1111 3FF
Midscale V
ref
/2 V
ref
10 0000 0000 200
Midscale – 1 LSB V
ref
/2 – 1 LSB V
ref
– 1 LSB 01 1111 1111 1FF
Zero 0 V 0 V 00 0000 0000 000
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Product Folder Link(s): ADS7950, ADS7951, ADS7952, ADS7953 ADS7954, ADS7955, ADS7956, ADS7957 ADS7958,
ADS7959, ADS7960, ADS7961