TI's two analog-to-digital converters aren't typical in any way. They are very high-resolution, 24-bit delta-sigma converters that cover just about every requirement for high-resolution de-signs. These two products provide data rates that can meet high-speed requirements and support the data rate to decrease the noise for low-speed requirements.
Because the converters boast noise performance at the slow-speed end or can be run at the high end, at 30 ksamples/second, they allow trade-offs between the two without concern. There's nothing to lose because, at high data rates, they can cycle through channels quickly with low latency, and at low data rates the noise performance stands out.
You must do your homework, however, and provide these high-resolution A/Ds with proper support circuitry to get the optimal performance. In particular, pay special attention to the reference and analog inputs. This is critical. You should bypass the voltage reference with low-ESR capacitors. Make these capacitors as large as possible to maximize the filtering on the reference. With the converters' outstanding performance, it is possible for the voltage reference to limit overall performance if it is not carefully selected. When using a standalone reference, make sure it is very low-noise and very low-drift.
The basic functions of TI's A/Ds are similar to other converters. They contain a mulitiplexer, a high-input impedance buffer and a programmable-gain amplifier (PGA) for measuring low-level signals used in strain gauges, or thermocouples, and for calibration with standard SCI-type interfaces. When measuring multiplexed inputs, these devices cycle between channels without data loss, which is pretty impressive. You can get over 18 bits at over 1 ksample/s without data loss.
Noise performance can be optimized by adjusting the data rate or PGA setting. As the averaging is increased by reducing the data rate, the noise drops correspondingly. The PGA reduces the input-referred noise when measuring lower-level signals.
The ADS1255 is packaged in an SSOP-20 and priced from $7.95 in lots of 1,000. The ADS1256 comes in an SSOP-28 and is priced from $8.95.