Datasheet

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2 Introduction
3 Analog Interface
3.1 Signal Conditioning
Introduction
The ADS7866 is a 12-bit, 200-KSPS serial-interface, analog-to-digital converter (ADC). The ADS7867 is a
10-bit, 240-KSPS serial-interface A/D converter. The ADS7868 is a 8-bit, 280-KSPS analog to digital
converter. The devices include a capacitor based SAR A/D converter with inherent sample and hold. The
serial interface for each A/D converter is controlled by the two signals chip select and serial shift clock.
The input signal is sampled with the falling edge of chip select, and serial shift clock is used as conversion
and the serial data output clock.
The devices operate from 1.2 V up to 3.6 V, and are available in SOT23 packages. Low power
consumption and a small size make these devices ideally suitable for battery powered, portable
applications. The ADS7866/67/68 devices are specified for operation from -40 ° C to 85 ° C.
The high level of the digital input to the device is not limited to device VDD. This means the digital input
can go as high as 3.6 V when the device supply is 1.2 V. This feature is useful when digital signals are
coming from other circuit with different supply levels.
The ADS7866/ADS7867/ADS7868 A/D converters have one analog input pin. A signal for the input pin
can be applied at connector P1, pin 2 (shown in Table 1 ), or applied to the center pin of SMA connector
J1. The input range of the converter is set by the power supply voltage applied at pin 1. For example, if
VDD = 3.6 V, then V
I
can range from 0 V up to 3.6 V or 3.6 V
PP
.
Table 1. Analog Input Connector
Description Signal Name Connector.Pin Number Signal Name Description
Reserved N/C P1.1 P1.2 ( + ) VIN
Reserved N/C P1.3 P1.4 N/C Reserved
Reserved N/C P1.5 P1.6 N/C Reserved
Reserved N/C P1.7 P1.8 N/C Reserved
Pin tied to Ground AGND P1.9 P.10 N/C Reserved
Pin tied to Ground AGND P1.11 P1.12 N/C Reserved
Reserved N/C P1.13 P1.14 N/C Reserved
Pin tied to Ground AGND P1.15 P1.16 N/C Reserved
Pin tied to Ground AGND P1.17 P1.18 N/C Reserved
Reserved N/C P1.19 P1.20 EXT_REF External Reference Input
The VIN input to the ADS7866, ADS7867, and ADS7868 should be driven with a low impedance source.
In many cases, an input driver is not necessary, but in those cases where the source impedance exceeds
200 Ω , using a buffer helps achieve the rated performance of the converter.
The amplifier circuit shown in Figure 1 is the buffer circuit used on the ADS7866/ADS7867/ADS788EVM.
This circuit consists of the THS4031, a high-speed, low noise, low distortion amplifier configured as an
inverting gain of one. The circuit shown Figure 1 is optimized to achieve the ac (i.e., SNR, THD, SFDR,
etc) specifications listed in the ADS7866/ADS7867/ADS7868 datasheets. Note the input circuit may
require adjustments to achieve best performance for specific applications.
The 15- Ω series resistor works with the capacitor, C20, to filter the input signal and isolates the amplifier
from the capacitive load. The 680-pF capacitor to ground at the input of the A/D works with the series
resistor to filter the input signal, behaves like a charge reservoir, and provides a short to ground for high
frequency noise and kickback currents when the device switches from hold to sample mode. This external
filter capacitor also works with the amplifier to charge the internal sampling capacitor during sampling
mode.
2 ADS7866EVM/ADS7867EVM/ADS7868EVM SLAU181 – July 2006
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