AD776
Obsolete
The AD776 is a 16-bit sigma-delta oversampled ADC, incorporating a 1-bit third-order modulator and digital decimation filter. An on-chip voltage reference...
AD776 on Analog.com
AD7767
Production
The AD7767/AD7767-1/AD7767-2 are high performance, 24-bit, oversampled SAR analog-to-digital converters (ADCs). The AD7767/AD7767-1/AD7767-2 combine the...
Datasheet
AD7767 on Analog.com
AD7765
Production
The AD7765 is a high performance, 24-bit sigma-delta (S-?) analog-to-digital converter (ADC). It combines wide input bandwidth, high speed, and performance...
Datasheet
AD7765 on Analog.com
AD7766
Production
The AD7766/AD7766-1/AD7766-2 are high performance,
24-bit, oversampled SAR analog-to-digital converters (ADCs).
The AD7766/AD7766-1/AD7766-2 combine the...
Datasheet
AD7766 on Analog.com
I would like to interface a photodiode transimpedance amplifier to a 24bit ADC (AD7765, AD776, AD7767 or similar). The largest source of error appears to be the drift of the transimpedance amp (TIA), rather than noise.
We prefer to use a linear amplifier, rather than a logarithmic amplifier. The topology is thus a linear TIA followed by either by a single ended to differential amp stage or by grounding one side of the ADC input if it provides better performance.
Assuming a 40 degree change in temperature, a 0.5uV/degree offset coefficient leads to a 20 uV input. If both the TIA and the SE to DE stage have a 0.5uV/degree drift coefficient, total drift could be as large as 40uV, which is too big for our application. For comparison a TIA gain of 5K in a 25kHz bandwidth has less than 2uV RMS noise, using a low noise TIA.
I would very much appreciate recommendations for TIA amplifiers, single-ended to differential amp, and input topology for achieving the lowest possible total error, within a -5 to 65 degree temperature range.
System parameters are:
Photocurrent range: 20nA to 1mA
Sampling rate: approximately 30 to 50 kilosamples/sec
Photodiode capacitance: 10pF
Thanks
Bernard Harris
Hi Bernard Harris,
ADA4528-X is a precision zero-drift operational amplifier. It as a maximum drift of 0.015uV/degC which could be used in a TIA configuration for your application. A dual ADA4528-2 can be configured to act as a single-ended to differential output converter at the same time. Please see picture below:
R2 is calculated based on the minimum VOH of ADA4528-2 from Tmin to Tmax for dual supply operation. Vp and Vn outputs will have peak output voltages of almost 2.5V each, which is suitable to the AD776X ADC inputs. R3 and R4 must be equal and could be set to 1kohms each. The feedback capacitor C2 stabilizes the TIA but limits the bandwidth to less than the unity gain bandwidth of AD4528-2. I think a C2 of 40pF should be able to give enough bandwidth and stability to counter the effects of the diode capacitance, trace capacitance, and amplifier input capacitance. I have simulated this circuit using Multisim, but can't guarantee its actual performance in the real world.
I hope this helps.
Regards,
Neil
