Datasheet

Data Sheet AD7734
Rev. A | Page 27 of 32
ANALOG INPUT’S EXTENDED VOLTAGE RANGE
The AD7734 output data code span corresponds to the nominal
input voltage range. The ADC is functional outside the nominal
input voltage range, but the performance might degrade. The
sigma-delta modulator was designed to fully cover a ±11.6 V
analog input voltage; outside this range, the performance might
degrade more rapidly. The adjacent channels are not affected by
up to ±16.5 V analog input voltage (Figure 8).
When the CLAMP bit in the mode register is set to 1, the
channel data register will be digitally clamped to either all 0s or
all 1s when the analog input voltage goes outside the nominal
input voltage range.
As shown in Table 16 and Table 17, when CLAMP = 0, the data
reflects the analog input voltage outside the nominal voltage
range. In this case, the SIGN and OVR bits in the channel status
register should be considered along with the data register value
to decode the actual conversion result.
Note that the OVR bit in the channel status register is generated
digitally from the conversion result and indicates the sigma-
delta modulator (nominal) overrange. The OVR bit DOES NOT
indicate exceeding the AIN pin’s absolute voltage limits
Table 16. Extended Input Voltage Range, Nominal Voltage
Range ±10 V, 16 Bits, CLAMP = 0
Input (V) Data (hex) SIGN OVR
11.60039 147B 0 1
10.00061 0001 0 1
10.00031 0000 0 1
10.00000
FFFF
0
0
0.00031 8001 0 0
0.00000 8000 0 0
–0.00031 7FFF 1 0
–10.00000 0000 1 0
–10.00031 FFFF 1 1
–10.00061 FFFE 1 1
–11.60040 EB85 1 1
Table 17. Extended Input Voltage Range, Nominal Voltage
Range 0 V to +10 V, 16 Bits, CLAMP = 0
Input (V) Data (hex) SIGN OVR
11.60006 28F5 0 1
10.00031 0001 0 1
10.00015 0000 0 1
10.00000 FFFF 0 0
0.00015 0001 0 0
0.00000 0000 0 0
–0.00015 0000 1 1
CHOPPING
With chopping enabled, the multiplexer repeatedly reverses the
ADC inputs. Every output data result is then calculated as an
average of two conversions, the first with the positive and the
second with the negative offset term included. This effectively
removes any offset error of the input buffer and sigma-delta
modulator.
However, chopping is applied only behind the input resistor
divider stage; therefore, chopping does not eliminate the offset
error and drifts caused by the resistors. Figure 24 shows the
channel signal chain with chopping enabled.
+
-
DIGITAL
INTERFACE
CHOPCHOP
f
MCLK
/2 f
MCLK
/2
BUFFERMULTIPLEXER
DIGITAL
FILTER
SCALING
ARITHMETIC
(CALIBRATIONS)
OUTPUT DATA
AT THE SELECTED
DATA RATE
AIN
BIASHI
BIAS
BIASLO
Σ−∆
MODULATOR
Figure 24. Channel Signal Chain Diagram with Chopping Enabled