Rev. B | Page 26 of 52
In buffered mode, the analog inputs look into the high impedance
inputs stage of the on-chip buffer amplifier. CSAMP is charged via
this buffer amplifier such that source impedances do not affect
the charging of CSAMP. This buffer amplifier has an offset leakage
current of 1 nA. In buffered mode, large source impedances
result in a small dc offset voltage developed across the source
impedance, but not in a gain error.
INPUT SAMPLE RATE
The modulator sample frequency for the AD7707 remains at
fCLKIN/128 (19.2 kHz at fCLKIN = 2.4576 MHz) regardless of the
selected gain. However, gains greater than 1 are achieved by a
combination of multiple input samples per modulator cycle and
a scaling of the ratio of reference capacitor to input capacitor. As
a result of the multiple sampling, the input sample rate of the
device varies with the selected gain (see Table 27). In buffered
mode, the input impedance is constant. In unbuffered mode,
where the analog input looks directly into the sampling capacitor,
the effective input impedance is 1/CSAMP × fS where CSAMP is the
input sampling capacitance and fS is the input sample rate.
Table 27. Input Sampling Frequency vs. Gain
Input Sampling Frequency (fS)
fCLKIN/64 (38.4 kHz at fCLKIN = 2.4576 MHz)
2 × fCLKIN/64 (76.8 kHz at fCLKIN =2.4576 MHz)
4 × fCLKIN/64 (76.8 kHz at fCLKIN =2.4576 MHz)
8 to 128
8 × fCLKIN/64 (307.2 kHz at fCLKIN = 2.4576 MHz)
The analog inputs on the low level input channels on the AD7707
can accept either unipolar or bipolar input voltage ranges with
respect to LOCOM.
The high level input channel handles true bipolar signals of ±10 V
for guaranteed operation.
Bipolar or unipolar options are chosen by programming the
B/U bit of the setup register. This programs the channel for
either unipolar or bipolar operation. Programming the channel
for either unipolar or bipolar operation does not change any of
the channel conditions, it simply changes the data output coding
and the points on the transfer function where calibrations occur.
In unipolar operation, the output coding is straight binary. In
bipolar mode, the output coding is offset binary.