Datasheet AD7880 (Analog Devices) - 7

AD7880
The AD7880 has two unipolar input ranges, 0 V to 5 V and 0 V
CLOCK INPUT
to 10 V. Figure 5 shows the analog input for the 0 V to 5 V The AD7880 is specified to operate with a 2.5 MHz clock con- range. The designed code transitions occur midway between nected to the CLKIN input pin. This pin may be driven directly successive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, by CMOS or TTL buffers. The mark/space ratio on the clock 5/2 LSBs . FS –3/2 LSBs). The output code is straight binary can vary from 40/60 to 60/40. As the clock frequency is slowed with 1 LSB = FS/4096 = 5 V/4096 = 1.22 mV. The same applies down, it can result in slightly degraded accuracy performance. for the 0 V to 10 V range, as shown in Figure 6, except that the This is due to leakage effects on the hold capacitor in the inter- LSB size is bigger. In this case 1 LSB = FS/4096 = 10 V/4096 = nal track-and-hold amplifier. Figure 10 is a typical plot of accu- 2.44 mV. The ideal input/output transfer characteristic for both racy versus clock frequency for the ADC. these unipolar ranges is shown in Figure 8.
2.5 OUTPUT CODE 2.0 111...111
A A
111...110
A
111...101
A
1.5 111...100
A A
1.0
A A A
NORMALIZED LINEARITY ERROR 0.5 000...011
A
FS 1LSB = 4096 000...010
A A
0.0 0.5 1.5 2.5 3.5 000...001
A
CLOCK FREQUENCY – MHz 000...000 1LSB +
A
0V FS – 1LSB
Figure 10. Normalized Linearity Error vs. Clock Frequency
V INPUT VOLTAGE IN TRACK/HOLD AMPLIFIER
Figure 8. AD7880 Unipolar Transfer Characteristic The charge balanced comparator used in the AD7880 for the Figure 7 shows the AD7880’s ± 5 V bipolar analog input con- A/D conversion provides the user with an inherent track/hold figuration. Once again the designed code transitions occur mid- function. The track/hold amplifier acquires an input signal to way between successive integer LSB values. The output code is 12-bit accuracy in less than 3 µs. The overall throughput time is straight binary with 1 LSB = FS/4096 = 10 V/4096 = 2.44 mV. equal to the conversion time plus the track/hold amplifier acqui- The ideal bipolar input/output transfer characteristic is shown in sition time. For a 2.5 MHz input clock, the throughput time is Figure 9. 15 µs.
OUTPUT
The operation of the track/hold amplifier is essentially transpar-
CODE
ent to the user. The track/hold amplifier goes from its tracking mode to its hold mode at the start of conversion, i.e., on the ris- ing edge of CONVST as shown in Figure 1.
111...111
A
111...110
A A
OFFSET AND FULL-SCALE ADJUSTMENT
In most Digital Signal Processing (DSP) applications, offset and A full-scale errors have little or no effect on system performance. A Offset error can always be eliminated in the analog domain by
100...101 – FS 2 1LSB –
A ac coupling. Full-scale error effect is linear and does not cause
100...000
A problems as long as the input signal is within the full dynamic
+1LSB + FS
A
011...111
A
– 1LSB 2
range of the ADC. Some applications will require that the input A signal range match the maximum possible dynamic range of the ADC. In such applications, offset and full-scale error will have
011...110
A
FS = 10V
A to be adjusted to zero.
1LSB = FS 000...001
A
4096
The following sections describe suggested offset and full-scale
000...000
A adjustment techniques which rely on adjusting the inherent off- set of the op amp driving the input to the ADC as well as tweak- ing an additional external potentiometer as shown in Figure 11.
0V V INPUT VOLTAGE IN
Figure 9. AD7880 Bipolar Transfer Characteristic –6– REV. 0