I think 1.2Us conversion time s not too fast.
this gives a a sampling frequency of 833333Hz
I think ARM7 LPC21xx can support up too 5 Mhz sampling rate.
Hm, ARM7 LPC21xx have converter frequency 5 Mhz. It is not "conversion time".
In doc (LPC213X) write "... more than 400,000 10-bit sample per second... "
and
...
One (LPC2131/32 ) or two (LPC2134/36/38 ) 8 channel 10-bit A/D converters provides a total of up to 16 analog inputs, with conversion times as low as 2.44 us per channel.
...
The biggest problem with a fast A/D on a processor is having enough processor performance to service the A/D converter and process the data. So you either need a very fast processor or a DMA capability or both!
Analog Devices (www.analog.com) makes some DSP's with 20MSPS converters on chip: ADSP2199x; and a 8051 microprocessor, ADUC812, with a 200kSPS converter.
TI also makes an inexpensive microprocessor family with A/D converters and a DMA controller: MSP430Fxxxx.
although I like the LPC21xx, the ADC needs 2.something usec and has a max samplerate in the range of 400ksamples/sec as sps pointed out already. All ADCs on the LPC2000 family seem to have the same specification except the number of input channels.
There are 1 MSPS ARM devices with 12-bit ADCs from Analog Devices. You can search for ADuC7022 as an example.
If a DSP is an option there are faster ADCs at a higher price.