ADS7864
www.ti.com
SBAS141A–SEPTEMBER 2000–REVISED MARCH 2005
When the input is differential, the amplitude of the
input is the difference between the +IN and –IN input,
or: (+IN) – (–IN). The peak-to-peak amplitude of each
input is ±1/2VREF around this common voltage. How-
ever, since the inputs are 180° out of phase, the
peak-to-peak amplitude of the differential voltage is
+VREF to –VREF. The value of VREF also determines
the range of the voltage that may be common to both
inputs (see Figure 22).
8000
7000
6000
5000
4000
3000
2000
1000
0
In each case, care should be taken to ensure that the
output impedance of the sources driving the +IN and
–IN inputs are matched. Otherwise, this may result in
offset error, which will change with both temperature
and input voltage.
2044
2045
2046
2047
2048
Code (decimal)
The input current on the analog inputs depend on a
number of factors: sample rate, input voltage, and
source impedance. Essentially, the current into the
ADS7864 charges the internal capacitor array during
the sampling period. After this capacitance has been
fully charged, there is no further input current. The
source of the analog input voltage must be able to
charge the input capacitance (15pF) to a 12-bit
settling level within two clock cycles. When the
converter goes into the hold mode, the input im-
pedance is greater than 1GΩ.
Figure 23. Histogram of 8,000 Conversions of a
DC Input
1.4V
Ω
3k
DATA
Care must be taken regarding the absolute analog
input voltage. The +IN and –IN inputs should always
Test Point
100pF
CLOAD
remain within the range of GND – 300mV to VDD
300mV.
+
VOH
VOL
DATA
TRANSITION NOISE
Figure 23 shows a histogram plot for the ADS7864
following 8,000 conversions of a DC input. The DC
input was set at output code 2046. All but one of the
conversions had an output code result of 2046 (one
of the conversions resulted in an output of 2047). The
histogram reveals the excellent noise performance of
the ADS7864.
tR
tF
Voltage Waveforms for DATA Rise and Fall Times tR, and tF.
Figure 24. Test Circuits for Timing Specifications
13
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