ADC1175: altough RT-RB delta 5 volts, after specific voltage, digital ports show very low voltage

Baris Erdogan

Part Number: ADC1175
Other Parts Discussed in Thread: ADS805

Hello,

I have a project that I get adc1175-20 mhz measured continuously a specific signal. If voltage max value is in about RT-RB, for example 4,30 volts, suddenly digital ports begin to show 0.30 volt or may be more or sometimes I read on digital outputs 30 it means 0.58 volt and it suspends that level. As if chip has max reference voltage is 4.30 volts and when Vin exceeds that limit, digital outputs show rest of what is left from 4.30. It is like mod operation in math. Or dividing operation in math. I have measured several times Vrt-rb and I am pretty sure that it was 5.00 volts or sometimes little bit more it has been changing -+0.01 volt.

All coupling caps like what is mentioned in datasheet and Vin gets driven by 10 mhz operational amp with voltage follower mode.

Vrt connects directly 5 volts Vrb connects directly to ground.

What would be the problem?

Thank you

Regards

over 5 years ago

0 Brian2001 over 5 years ago

TI__Intellectual 2750 points

Hello Baris,
I have sent over your question to an engineer that works with this device.
Regards,
Brian

0 Jim Brinkhurst1 over 5 years ago in reply to Brian2001

TI__Mastermind 43910 points

Hi Baris

On the ADC1175 the Reference Voltage Delta (VRT - VRB) has an allowable range of 1.0V to 2.8V (see datasheet bottom of page 6).

If you are using a Reference Voltage Delta of 5V that is too large, and the ADC is not expected to operate properly under those conditions.

If you need an ADC that can handle a signal from 0V to 5V you should consider the ADS805. Figure 7 of the ADS805 datasheet shows how to configure the device for this usage.

Best regards,

Jim B

0 Baris Erdogan over 5 years ago in reply to Jim Brinkhurst1

Prodigy 180 points

Hello Jim,

I want voltage delta be fixed 2.00 volts or may be at most 2.2 volts. In this case will voltage divider method work for making reference delta lower or is that the only way like datasheet figure 21 in page 18

Thank you
Regards
Baris

0 Jim Brinkhurst1 over 5 years ago in reply to Baris Erdogan

TI__Mastermind 43910 points

Hi Baris

The circuit shown in the ADC1175 datasheet Figure 20 will provide a fairly accurate voltage delta of 2.0 V. In that circuit Vrt=2.6V and Vrb=0.6V. The exact values will be dependent on the accuracy of the voltage reference and divider resistors used.

If you need a different voltage delta (larger or smaller than 2.0V then the circuit in Figure 21 is the recommended approach. Again the absolute accuracy is dependent on the components used. If very tight tolerances are required you may need to incorporate some adjustment of the resistor values which can be set during product testing.

Best regards,

Jim B

0 Baris Erdogan over 5 years ago in reply to Jim Brinkhurst1

Prodigy 180 points

Hello Jim,

I applied the circuit in where datasheet figure 20, I was able to fix voltage delta 2.13 volts and set signal attenuation according to that value. It seems to be working fine.

Thank you
Regards
Baris

0 Baris Erdogan over 5 years ago in reply to Baris Erdogan

Prodigy 180 points

Hello,

I don't know Who's is responsible for case's edit but subject of this case is required to be corrected like

"Showhello very low" to "show very low"

Thank you
Regards
Baris

0 Jim Brinkhurst1 over 5 years ago in reply to Baris Erdogan

TI__Mastermind 43910 points

Hi Baris
I'm not able to edit the title of the thread. I will ask the E2E forum support team if they can correct it.
Best regards,
Jim B

0 Jim Brinkhurst1 over 5 years ago in reply to Jim Brinkhurst1

TI__Mastermind 43910 points

Hi Baris
The subject has been corrected.
Best regards,
Jim B

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ADC1175: altough RT-RB delta 5 volts, after specific voltage, digital ports show very low voltage